I have never worked specifically on this fascinating topic, however I briefly discussed this topic in my book Tigers, Lions and Humans. To me sexuality in social species, apart from the function of propagation, has a very important function of social coherence and bonding. With this we can explain the great number of animal species that use homosexuality as a social glue between the same sex animals in a group.
For me humans are by nature bisexuals, but existing cultural prejudices and religious bans strongly affect (mostly limit) their sexual orientation and desires. I discuss how homosexuality was used in human history to strengthen the morale of early fighters.
For futher reading you can see:
- "Human sexuality, homosexuality and bisexuality, or who can defeat 300 Spartans?": chapter on this subject from my book Tigers, Lions and Humans: History of Rivalry, Conflict, Reverence and Love.