The word "sex" often refers specifically to coitus (penile–vaginal intercourse), but various other activities are usually also considered sex, such as anal sex, fellatio, cunnilingus, and mutual masturbation. Sex is typically between two people, but then there's group sex. Typically the participants are of opposite sexes, but sometimes they aren't. Often the participants are roughly the same age and consider themselves to be in a passionate and intimate relationship although they aren't related by blood, but sometimes they're of vastly different ages or they don't even know each others' names or they're siblings. Usually sex is consensual, but sometimes it isn't: a party who doesn't really want to have sex has somehow been coerced into it. And if one of the participants is human, then usually they all are, but sometimes animals are included as well. Finally, masturbation is clearly a close relative of sex but is generally considered to be distinct from it for the simple reason that only one party is involved.

Are these practices ethical? Are they reasonable things to do? Some, to be sure, are not. Rape is unethical, almost by definition, and quite dangerous to victims, as discussed in a later chapter. People occasionally kill themselves masturbating: Blanchard and Hucker (1991) summarize 117 cases of autoerotic asphyxia in two Canadian provinces within fourteen years. And if media violence can make people more violent (it can, even if the likes of Jack Thompson have caricatured this view: see, for example, Anderson et al., 2010), then presumably violent sexual roleplaying can as well.

That said, I think these potentially problematic practices are the exceptions when it comes to sexual behavior. Most of the excuses people come up with for condemning certain kinds of sex don't hold water. Sex can cause unwanted pregnancies or transmit deadly diseases, but pregnancy is easy to avoid, and the risk of infection can be reduced to a low level. Sex can be disgusting (to only one person or to the vast majority of the human population), but simply because something is disgusting doesn't mean it's destructive or dangerous. Sex can violate religious codes, but religious codes by nature rest on belief in supernatural phenomena and are therefore indefensible from an empirical perspective. Sex can even violate prejudices that may well be innate to humans because of their value for evolutionary fitness, like the taboo against incest, but prejudice alone is no justification. For example, Haidt, Bjorklund, and Murphy (2000) told subjects a short story in which two siblings copulated consensually and with a condom. Less than 11 of the 30 subjects judged this to be acceptable, although 23 subjects together made 38 statements that they were dumbfounded, that is, statements "to the effect that they thought an action was wrong but they could not find the words to explain themselves".

This is not to say that blanket relativism is necessarily correct, either:

It has often been asserted (by some sexologists, sex educators, and sex therapists) that all sexual behaviours are approximately equal. This assertion springs from the confluence of political prejudices, weak research methods, and unquestioning devotion to the received lore. [Brody, 2006, p. 393]

Even if I wouldn't put it as strongly as Brody, I agree with his point that it is unjustifiable and dangerous to assume that every flavor of sex is equal. What we need, if we want to make prescriptive claims about sexual behavior—that is, claims about what people should do—is true experiments (clinical trials, of a sort) that randomly assign regimes of sexual behaviors and observe the consequences, for behavior as well as for health. Only experiments with random assignment can provide direct evidence of causation or the lack thereof. By and large, such research does not currently exist. In the meantime, we should suspend our scorn. It is inappropriate to condemn that to which we can't attribute potential harm. Hence the title of this chapter.

I know of one study in which sexual behavior, or at least the frequency thereof, was randomly assigned to subjects. Loewenstein, Krishnamurti, Kopsic, and McDonald (2015) observed 64 opposite-sex couples for three months, asking 35 of them to have sex twice as often as they usually did. Experimental couples indeed reported having sex more often than control couples: their mean frequency was greater by 0.4 occasions per week. But experimental couples had the lower mean score on a 14-point scale of mood, by 2.6 units. These results suggest that, despite observational findings that more sexually active people are happier (e.g., Blanchflower & Oswald, 2004), sex does not make people happy.