2-gay help raise sibling from the same gene pool, overall helping it.
3-gay can reproduce - 10% have biological children in the US-
4-gay come from "epigenetic": -hormone from mother before birth-
I wonder if @theAdvancedApe didnt already post something along the line.
I hate to throw cold water on anybodies thinking but a lot of gay men have children.
Biological children they had because they had sex with a lady.
Until recently gay men were usually married and giving it the old college try at their marital duties. (possibly while thinking of the men they would be with later)