The examples and perspective in this article may not represent a worldwide view of the subject. (August 2020)
Casual sex is sexual activity that takes place outside a romantic relationship and implies an absence of commitment, emotional attachment, or familiarity between sexual partners. Examples are sexual activity while casually dating, one-night stands, prostitution or swinging.
A one-night stand is a single sexual encounter between individuals, where at least one of the parties has no immediate intention or expectation of establishing a longer-term sexual or romantic relationship. Anonymous sex is a form of one-night stand or casual sex between people who have very little or no history with each other, often engaging in sexual activity on the same day of their meeting and usually never seeing each other again afterwards.
"Friends with benefits" and "booty calls" describe a situation in which a person has sex with someone they generally consider a friend or someone they are fairly close to. They are not in an exclusive romantic relationship. The involved parties may have a degree of emotional attachment but do not want, for whatever reason, to have "strings attached". However, a 2011 study published in The Journal of Sex Research found that two out of five single women and one out of five single men in "friends with benefits" relationships hoped that their relationship would eventually turn into a full-fledged romance. This stands in contrast to swinger couples who are already in long-term relationships and are only seeking compatible friends with whom they can engage in recreational sex.
A "hookup" (colloquial American English) is a casual sexual encounter involving physical pleasure without necessarily including emotional bonding or long-term commitment; it can range from kissing (for example, making out) to other sexual activities. Hooking up became a widespread practice among young people in the 1980s and 1990s. Researchers say that what differentiates hooking up from casual sex in previous generations of young people is the "virtual disappearance" of dating, which had been dominant from the postwar period onwards. Today, researchers say, casual sex rather than dating is the primary path for young people into a relationship. With the use of apps becoming more common it has become easier to hook up or meet others for sexual activity. With students, studies have shown that the group most likely to engage in casual sex is white middle or upper-class heterosexuals. Black and Latino students are less likely to hook up, as are evangelical Christian students and working-class students. Data on gay and lesbian students show mixed results, as some research shows that they engage in hookups at the same rate as heterosexual students, while others suggest that it occurs less due to college parties not always being gay-friendly, as most hookups occur at such gatherings.
A study of hookup culture at the University of Iowa found that waiting to have sex does not contribute to a stronger future relationship. Instead, the study found that what mattered most was the goal individuals had going into a relationship. Individuals who started by hooking up tended to develop a full relationship later if that was their goal going in. Another survey revealed that the number of first dates that most college-aged students have been on is about half of the number of hookups they've had. As of the recent decades, it has become a common dating practice for people to candidly talk about their casual sex encounters and personal views on the subject as early as the first date or meeting. Another study showed that condoms were used in only 69 out of every 100 penetrative sex hookups.
Swingers in the lifestyle engage in casual sex with others for a variety of reasons. For many, an advantage is the increased quality, quantity and frequency of sex. Some swingers engage in casual sex to add variety to their otherwise conventional sex lives or for curiosity. Swingers who engage in casual sex maintain that sex among swingers is often more frank and deliberative and therefore more honest than infidelity. Some couples see swinging as a healthy outlet and a means to strengthen their relationship. Others regard such activities as merely social and recreational interaction with others. A swinger party or partner-swapping party is a gathering at which individuals or couples in a committed relationship can engage in sexual activities with others as a recreational or social activity. Swinging can take place in various contexts, ranging from a spontaneous sexual activity at an informal social gathering of friends to a regular social gathering in a sex club (or swinger club), private residence, or other pre-arranged location such as a hotel, a resort, or a cruise ship.
In the United States, in the 1920s, "petting parties", where petting ("making out" or foreplay) was the main attraction, became a popular part of the flapper lifestyle. Because automobile ownership boomed and the number of options for dating venues, such as movie theaters and jazz halls, increased significantly during this time, parental involvement in the courtship process began to decrease. This allowed for casual hookups to become a more common occurrence in the teen and young adult dating experience. Automobiles began to be referred to as "brothels on wheels" for this reason, [clarify] The young adults of the 1920s found the Victorian era sexual script and moral codes to be oppressive, and thus the liberated flapper and vamp personas were born. The emerging movie industry furthered progress in the rebellion against Victorian era morals because films started depicting women owning their sexuality, a trend that has continued into current cinema. Women became sexually liberated through their adopted flapper and vamp identities, as well as through progress made in contraceptive and family planning methods.
During the sexual revolution in the United States and Europe in the 1960s and 1970s, social attitudes to sexual issues underwent considerable changes. The advent of "the pill" and other forms of birth control, the Women's Liberation movement, and the legalization of abortion in many countries are believed to have led to a wider practice of casual sex. This is also due, in part, to the younger generation's rejection of their parents' dating and matrimonial ideals, and the rise of college party culture. In response to this insurgence in the 1960s, single women were denied access to birth control pills by their healthcare providers. This type of pushback is consistently observed throughout studies of the evolution of American sexual morals and beliefs. Younger generations are encouraged by their elders to engage in sexual activity only if it is within the bounds of marriage and is for procreative purposes.
In the United States, one-time sexual encounters among college-aged students are growing increasingly common; nearly 70% of people in this age group have partaken in casual sex at least once because of their newfound adult identities and freedom to explore their sexualities.
Data from 2007 to 2017 indicates that there has been a decline in casual sex among young adults. A 2021 study attributed this decline to reduced alcohol consumption, increased video gaming, and a larger share of young adults living with their parents.
Casual sex is a high-risk activity during COVID-19 and research has shown that many people are limiting contact with sexual partners outside their social bubbles. When public health guidelines exist, the suggestions are fairly consistent: 'solo sex' and sex with a housemate (spouse, roommate, etc.) are safest and masks and disinfection (hand washing, shower, etc.) are essential for sex with a partner outside your social bubble. 
A study in 2019 found that 23-33% women surveyed dated men for a free meal termed "foodie call".
Research suggests that as many as two-thirds to three-quarters of American students have casual sex at least once during college. On college campuses, casual sex may occur almost anywhere. The majority of hookups happen at parties. Other common casual sex venues are dorms, frat houses, bars, dance clubs, cars, and in public places or wherever is available at the time.
Collegiate holidays and vacations, especially spring breaks, are times when undergraduates are more likely to purposely seek out casual sexual encounters and experiment with risky behaviors. This is due to the availability of alcohol and uninhibitedness of spring break venues. One study reported that about 30% of collegiate spring breakers, whether they are in a monogamous relationship or not, have penetrative sex with a person they meet during their break. A 1995 study of Canadian students who had traveled to Florida for spring break, found the key elements of a spring break vacation to include a group holiday with friends traveling and rooming together, a perpetual party atmosphere, high alcohol consumption, sexually suggestive contests and displays, and the perception that casual sex is common. Overall, there was a perception that sexual norms are far more permissive on spring break vacation than at home, providing an atmosphere of greater sexual freedom and the opportunity for engaging in new sexual experiences. Of the 681 students who completed a questionnaire after the break, 15% of males and 13% of females had engaged in casual sex during the break. Nearly 61% of men and 34% of women who had casual sex over the break had slept together within 24 hours of meeting.
Men and women are found to engage in very similar casual sex conducts, despite popular social beliefs. Most young adults in this age group believe that their peers are having a higher frequency of casual sex than they actually are, and this is due to vocabulary choice. For example, using the term "hookup" denotes that the sexual activity, whether it is vaginal sex, oral sex, or sexual touching, is casual and between unfamiliar partners. However, it is vague and does not detail what specific sexual activities occurred. This is especially distorting towards others' impressions because 98% of college hookups involve kissing, 81% of hookups involve more than kissing, and only 34% of hookups involve penetrative sex. Studies have also linked this common misperception of peer hookup activity to media and pop culture portrayals of casual sexual encounters. Television and movies project distorted depictions of casual sex because they also commonly portray people who have just hooked up as emotionally satisfied and physically pleasured while simultaneously emotionally detached, which is not always the case.
Attitudes to casual sex range from conservative and religious views, the extreme of which may result in imprisonment or even capital punishment for sexual relations outside heterosexual marriage, to liberal or libertarian (libertine) views, the extreme of which is free love.
In some Muslim-majority countries, such as Saudi Arabia, Pakistan, Afghanistan, Iran, Kuwait, Maldives, Morocco, Oman, Mauritania, United Arab Emirates, Sudan, and Yemen, any form of sexual activity outside marriage is illegal.
Most religions disapprove of sex outside marriage (see religion and sexuality), and the consequences range from very serious to none. Also, marriage is defined in quite different ways in different cultures, for example, with "short-term marriage" (see Nikah mut'ah) a cover for prostitution, or polygamy.
Although some religious views look upon casual sex negatively, individual views on casual sex vary based on personal traits such as autonomy. Religious people and non-religious people mostly have similar feelings when it comes to having casual sex, with a bigger difference in attitude taking place between men and women.
Many specialist online dating services or other websites, known as "adult personals" or "adult matching" sites, cater to people looking for a purely sexual relationship without emotional attachments. These can provide a relatively anonymous forum where people who are geographically close but in totally separate work and social circles can make contact.
Tinder is a free smartphone dating app that boasts over 10 million daily users, making it the most popular dating app for iOS and Android. In this app, users can either swipe right (which indicates interest) or swipe left (which indicates disinterest) on other users in hopes of matching each other. If both users swipe right on one another, they are a match, and messaging can be initiated between parties. This app is used for a variety of reasons, one of which is casual hookups. Men are more likely than women to use Tinder to seek out casual sexual encounters. This is attributed to men, in comparison to women, placing sexual pleasure at a higher level of importance and using social networking sites to fulfill this need. However, overall, more users are motivated to use it to find romance rather than sex. Despite this, there is a social concern as some believe that the app encourages hookups between users.