In fact, the idea of sexual exclusivity – of humans being concerned with genetic lineage and trying to avoid raising another man’s child – is a relatively recent development, evolutionarily speaking.
Up until about 10,000 years ago (a not even a blink of the eye, evolutionarily speaking), humans lived in small disparate communal groups with no real concept of individual ownership or even parentage.
Yes, there will be the inevitable quipster who says something about the stereotypical promiscuity of gay males, but gay men aren’t instinctively trying to spread their genes to as many females as possible.
Similarly, lesbians aren’t seeking out alpha males for their genetic superiority.
Sexual relationships weren’t a question of monogamy or harem-like structures but polygynous and polyandrous.