Perhaps some sort of animal had come in contact with a disease that only killed or negatively impacted the females of the species, did that lead to increased homosexuality in the males of the species?
I always like animal to human comparisons and I'm just wondering if I can link the rise of homosexuality to the decrease in female quality by using some sort of animal comparison.
The Mouse "Utopia" experiment is the closest I can think of. Not the same but might be somewhat similar.