There is an African saying attributed in our modern times to the Nigerian Igbo culture the proverb "
Ora na azu nwa" which means "It takes the tribe/village to raise a child."
Non-procreating adults in the tribe who would hunt and gather yet not add to the burden of the population by procreating aids the survival of the tribe and the species.
http://www.medicalnewstoday.com/articles/111843.php sets forth a new theory for male homosexuality, tying it in with higher fertility of mom.
In essence what it is saying that the more fertile a woman is the greater her chances of having homosexual male offspring. There have been several studies on the issue, and they more or less agree.
This suggests that in order to get around high infant mortality rates, women who are able to produce more offspring is an asset to the species.
Considering how many gay males are drawn to child tending professions, teachers, nanny, etc, we have to wonder if there is not a natural instinct in gay males to be tenders of children. It is no big secret that gay males tend to be more sensitive, more in touch with their emotions. This leads to gays being more prone to developing a 'maternal' attachment to the offspring of others.
Straight men are far more competitive when it comes to the offspring of others. Being a 'step father' is more difficult for straight men than for gay men on many emotional levels. This goes back millions of years as males competed with each other to insure that their genetic material survived. Gay males are less competitive in this area are far more willing and emotionally able to become attached to a 'strangers' child and become a foster parent.
Gay males tend to be 'hardwired' toward rearing other people's children. While that serves little purpose in our modern world where we have adopted the nuclear family, way back when we were hunter gather's as part of a tribe, that ability was advantageous to the survival of the tribe, thus survival of our species.
I suspect we will find similar links to the survival of our species when it comes to lesbians. Lesbian women tend to be more daring, more risk taking and quicker to charge forward in defense. In a time when the women folk were left in the camp while the menfolk went off to hunt for days at a time, having stronger, more protective women left behind with the women made perfect sense.
Again we see a strong child rearing 'willingness', lacking the willingness to pair off with the opposite gender in order to create an offspring.
This means that the children in the tribe had more parents, willing and able to defend and provide for those children as if the children were they very own biological copies.
Considering how helpless human children are, unable to get up and walk in the first moments after birth, requiring excessive amounts of protection, care, feeding and tending for years before being able to tend to its own needs, having more adults willing and able to meet its requirements becomes a survival tool not just for the individual but for the whole tribe and the whole species.