A hundred years ago, to admit to a one-off sexual encounter with a stranger would be to lose one’s social position instantly. These days, we’re a little more pragmatic. Few and far between are the couples who wait until marriage to be sexually intimate, and those who do are often acting on the basis of strong religious faith.

We’ve written before about average numbers of sexual partners over a lifetime, which are roughly 6 for men and 4 for women – although the gap between may be narrowing, driven by movement on both sides. But what do those numbers mean for our health and happiness?

We know that men tend to want considerably more sexual partners than women. This is unsurprising: women are more likely to feel miserable after casual sex, and more likely to engage in it because they feel unhappy or pressured. The orgasm gap, in which men typically achieve sexual satisfaction from casual sex where women are left wanting, is doubtless one of several factors at play.

The difficulty for women is that sexual intimacy is, for the majority of couples, intricately related to relationship intimacy in general. As relationship intimacy increases, so does sexual behaviour. Social norms dictate that emotional closeness in romantic relationships leads to sexual interaction –– although men typically expect sex in relationships earlier than women do, and are more likely than women to want sex without emotional intimacy at all.

This helps explain part of why casual or early-relationship sex is more likely to make women unhappy than it is men: women “give in” to male desire for sex more often than vice versa. One study of 25-35 year old Americans found that more than half of women rated their interest in casual sex as 0-1 out of 10, and two thirds rated it as 3/10 or below. Safety concerns, fear of objectification and humiliation, lack of emotional trust, and low expectations of sexual satisfaction were all given as reasons for the lack of interest, and more than 1 in 5 women spontaneously reported stories of past sexual assault.

Of course, there's a distinction to be drawn between casual sex and sex early on in a potential relationship. Both men and women see the latter as an important way to test out compatibility, even if they disagree about when in a relationship it should happen. Equally, no article about sex would be complete without mentioning its benefits. Frequent sex, whether in a relationship or not, offers many benefits commonly associated with exercise, such as lower blood pressure and better heart health – but also a significantly lowered risk of breast cancer in women and prostate cancer in men, improved immune system function, and significantly extended life expectancy.

The psychological benefits of sex are almost as great: frequency of sex is a significant predictor of satisfaction with mental health for both men and women, and it’s associated with better ability to express our feelings. It also correlates with psychological maturity and ability to relate to the opposite sex. (It’s worth noting that many of these benefits are unique to penile-vaginal intercourse, and sometimes require that the sex is unprotected.)‍

All things being equal, sex has many benefits for human health and happiness. But these benefits are nullified or countermanded when the sex is unwanted. Women are particularly, though not uniquely, at risk of having sex they don’t want and of feeling pressured into sexual activity. This pressure is often social, not physical, and is not always the same as direct sexual coercion.

Those subject to unwanted sex, even in relationships they do want, experience worse physical, psychological, and sexual health outcomes than those whose sexual activity is wanted. Those bad outcomes include depression, anxiety disorders, PTSD, sexually transmitted diseases, unwanted pregnancies, and increased likelihood of being exposed to sexual abuse in the future.

Several people are credited with saying that sex is like pizza: when it’s good, it’s amazing, and when it’s bad... well, it’s still pretty good. In reality, sex is more like a delicate preparation of chicken or fish. When it’s good, it’s amazing, but when it’s bad, the risks are substantial. (And for some people, chicken and fish aren’t on the menu anyway.)

The complicated fact is that whether casual sex is good or bad for you depends mostly on the circumstance. But circumstance is to an extent controllable, and we have a pretty good idea of which situations lead to positive outcomes. Casual sex is most likely to result in satisfaction when several of the below criteria are met:

• You know the person you're sleeping with
• You're sober, or at least not drunk
• You're sexually satisfied in the process
• You're a man (and, crucially, not a teenage girl)
• You prevent unwanted pregnancies and STIs
• You're confident and happy with your body and sexuality

If you can hit most of those, the odds are good that you'll have fun. After all, most casual sexual encounters are reviewed positively by those who have them. And, perhaps more encouragingly, there's evidence that relationships that start with sex are just as happy and enduring as those that delay it.