Unfortunately for humans, evolution has taken us down a different path from the funtime orgy we'd all be having if we were bonobos. Humans, compared to bonobos, exhibit significant adaptations to monogamous pair bonding, or at most mild polygyny, and use a variety of tools (including violence) to control the sexuality of their partners and other conspecifics.
We're not hippie apes; we're jealous apes. Human heterosexual sexuality is simply not analogous to bonobo heterosexual sexuality.
But the question I have not seen addressed, much less resolved, is whether human female-female sexuality has played a significant role as a cooperative mating strategy.
Evidence Suggestive of a Role for Female Cooperative Sexuality
The first piece of evidence for an adaptive role for female-female sexuality is the existence of female-female sexuality in humans. We can do better than the existence case, though: there is evidence that the capacity to be sexually aroused by women is near-universal in human women. Female sexual response is much more bisexual (and even bestialist!) than male sexual response, regardless of self-identified orientation.
Second, there is evidence that life history affects female sexuality in a manner consistent with the predictions of evolutionary psychology. One study found that lesbians with prior hetero experience were more likely than heterosexual women to have experienced physical and sexual abuse by men. To the extent that heterosexual sexuality is found to be dangerous, through early rape or severe abuse, one might be expected to look for a different strategy to survive and reproduce than heterosexual pair bonding.
Third, human males are highly aroused by images of female-female sexuality. In that vein, nominally heterosexual women often engage in public situational bisexuality in the context of (presumably) heterosexual mate selection, such as sticking their tongues down my throat on the dance floor at a heterosexual club. Female-female sexuality is more widely displayed in heterosexual contexts than male-male sexuality. (I have only rarely observed situational male-male sexuality in a largely heterosexual context, and in all cases a number of females had expressed their arousal over, and approval of, this type of sexuality.)
Fourth, female-female sex is "costless" in evolutionary terms, whereas female-male sex is very costly to females. Being costless, female-female sexuality fails to cry out for an explanation the way fitness-damaging behaviors do. A low-cost tool is one we might expect to see used as part of a variety of strategies. This post was inspired in part by Dain Fitzgerald's An Ev Psych Standard Gets a Twist, in which he relates the curious case of Terri Conley. Conley, in a serious of papers, attempts to challenge Sexual Strategies Theory (i.e., that women aren't as interested as men in casual sex because duh pregnancy) for a variety of stupid reasons. The stupidest alleged attack on SST is that women are shockingly willing to engage in casual sex WITH ANOTHER WOMAN, just not with a man. She (and the popular press) attribute this almost completely to women getting greater sexual satisfaction from women as opposed to men.
While appealing to male fears of sexual inadequacy is totally hilarious, the proposition that a girl is going to sexually satisfy you better than a guy is hardly a given. There are two much more salient features of girl-on-girl action, and they both relate to it being relatively costless.
One aspect of costlessness is material: girls don't fertilize you. This has two consequences: first, you don't incur the costs of pregnancy and childrearing; sex can be just sex. Second, you don't incur the jealousy (or reputational) costs inherent to a fertilization risk; there's no risk of paternity confusion, so none of the negative consequences that might flow from paternity confusion need be realized.
The second aspect is more purely one of signalling. Men are easy; girls are hard. As Eliezer Yudkowsky has noted, the size of a nerdy female's harem is limited only by the stress tolerance of her clit ring. Sleeping with men is not an indication of high mate quality; it may be the opposite, given the value of a woman's reputation for chastity (at least sperm chastity). But having sex with women is an indication of quality and status; I for one am about four thousand times more likely to brag about an all-girl fivesome in the shower of Senior House than about any sexual act involving males, even though I'm predominantly heterosexual. In a dyadic situation, female-female sexuality may even signal to males that sexual access to one means sexual access to both.
Polygyny and Its Discontents
Does the idea of cooperative female-female sexuality conflict with the high levels of conflict (and presumably low levels of scissoring) observed among co-wives in polygynous societies? From the abstract of the 2005 paper "Co-Wife Conflict and Cooperation":
Conventional wisdom holds that the polygynous family system is as sexually and emotionally satisfying as a monogamous one. Ethnographic accounts of 69 polygynous systems, however, provide compelling evidence that the majority of co-wives in a polygynous family prefer pragmatic co-operation with one another while maintaining a respectful distance. Moreover, there is often a deep-seated feeling of angst that arises over competing for access to their mutual husband. Co-wife conflict in the early years of marriage is pervasive, and often marked by outbursts of verbal or physical violence. Co-wife conflict may be mitigated by social institutions, such as sororal polygyny and some form of "social security" or health care. Material wealth may be divided more or less equally, but as a husband's sexual attention (a primary source for increased fertility) and affection cannot always be equitably distributed, there is ongoing and contentious rivalry among co-wives. [NSFW link mine.]
The high-conflict, high-stakes environment of polygyny is not a happy lesbian love-fest. But this environment might be just the place for pockets of sexual cooperation between females (what better hack to get the patriarch's attention?); do female-female sexual relations exist in these societies, and under what circumstances? I am not aware of the answer to this question.
But if girl-on-girl bonding sex isn't used in modern polygynous societies, that doesn't mean it isn't used in other contexts relevant to our environments of evolutionary adaptedness. "Unicorn polyamory" is the pejorative term used to describe a male-female couple in search of a second female to complete a three-person relationship; such females are located about as often as is the concept's namesake. Unicorn polyamory, in my view, is similar in sexiness to the idea of entering somebody else's polygynous marriage: not fucking very, mister patriarchy. There's not much advantage to banging your co-wife, unless the two of you can out-compete the rest of the co-wives that way. Similarly, there's no advantage to sharing a male with another female if you could reliably get your own, assuming high paternal investment.
But when the female-female sexual cooperation comes first, the female dyad is more powerful, and sexier, than either female individually - perhaps more than both put together. I would expect cooperative female-female sexuality, to the extent that it exists, to be geared toward the formation of female-female sexual dyads for the purposes of (a) attracting the attention and investment of high-quality males and (b) pooling resources to raise children safely.
Happy Valentine's Day!