The Problem with Defining “Sex”
The study, conducted by researchers at Indiana University, looked at the reported sexual behaviors of 32,831 gay and bisexual men via online survey. The survey listed 12 sexual behaviors; romantic cuddling, kissing, solo masturbation, masturbation by a partner, genital-genital contact, giving or receiving oral sex, penetrative or receptive anal sex, and giving or receiving oral anal stimulation. For the participants' most recent sexual encounter with another man, the most common reported sexual behavior was giving oral (75%). Kissing came next (74.8%), followed by receiving oral (73.4%). Only 35.5% of men reported being on the receptive end and 33.8% on the penetrative end of anal sex, although younger men were more likely to report receptive anal as their most recent sexual activity than older men and men 30-39 were most likely to be inserting anally. Of course, most of the sexual encounters reported involved a range of activities, for example kissing and mutual masturbation and mutual oral.
A smaller study of lesbian sexual behavior from 1992 found a similarly diverse repertoire of sexual behaviors. The survey of frequent and occasional same sex behavior for 1218 gay and bisexual women reveals that the most common frequently engaged in behavior was digital penetration of the vagina (84%). Oral stimulation (72%) and mutual masturbation (71%) were the next most common, with genital-genital occurring at slightly lower rates (50%). Vaginal penetration with a toy, fisting, anal penetration, and anal-oral stimulation were the least common sexual activities reported (16%, 16%, 13%, and 10%, respectively). The data is for repeat activities, rather than last-time encounter activities, so it lacks the combination platter details of the study on men, but it's probably safe to assume that most same-sex encounters between women involve multiple activities as well.
The question is, which activities count as "sex," or at what point does a combination of sexual activities similarly count. Is mutual masturbation and genital contact sex? Or is it oral that bumps the encounter up to, "we had sex," from, "we...did...stuff...involving penises/vaginas." It's an important question because if the definition of sex is too strict then the result can be that people engage in activities they don't really enjoy because they feel that that's what their supposed to do. It's telling, for example, that the most commonly reported sexual behavior between women is digital penetration of the vagina rather than, say, clitoral stimulation (actually, there is no specific question regarding clitoral stimulation, which I'd consider a major flaw of the study, but I presume it's covered under oral sex and mutual masturbation). This despite the fact that more women get off from clitoral stimulation than vaginal (not that getting off is the only point of sexual activity, but it's a helluva big one for most people). The higher rates of receptive anal sex among younger men may also reflect an idea of what sex should be rather than what the participants want it to be.
Unfortunately neither study investigated the reasons for engaging in specific activities, but given the culture dominance that penetrative sex receives, it's not hard to imagine that at least some of the research subjects were responding to cultural influences when making their sexual choices. (For an example, check out this recent Savage Love article, which features a gay man who worries that his distaste for anal is a sign that there's something wrong with him that will doom him to a life without a partner). This is a problem too, though usually to a lesser extent, for heterosexual couples - sex, the act itself, isn't whatever sexual activity you like, it has a somewhat specific cultural definition (i.e. penetration, preferably of the vagina by a penis but in a pinch only one of those organs needs to be involved) and since that one version of sex is supposed to be the be-all end-all of a sexual relationship, many people treat it as the primary form of sexual engagement and close themselves off to other forms of sexual expression that they might find more fulfilling. Don't get me wrong, I love vaginal penetration myself, it's a personal favorite and there are plenty of other people out there, maybe the majority, who do indeed find penetration to be the most enjoyable form of sex, but it isn't a one-size-fits-all proposition. Of course people have been arguing for years that we should get away from this need to define sex so specifically, but maybe seeing that other people do indeed engage in many activities that they consider sex, as the study on gay men indicates, will help more people feel comfortable defining sex for themselves. Which is really how it should be - not a single definition that everyone uses but a personal one. Sex is whatever you want to do when you get physically intimate with another person.