I dont actually think thats what happened.
The whole genetic modification thing was less of a "everyone starts gay" and more of a "people will fuck whoever they can to relieve stress" from what I could tell.
And as far as the outgrow it thing, i dont think thats true.
Spoilers for end of show as to why:
The whole "dont be gay but instead make babies" i think was actually a pretty big social commentary on Japan and their views of Homosexuality, as well as the whole "everything is controlled by an unseen force of elders, and if you go against them they will kill you" idea. Again, a pretty big commentary on Japan.
It was still a great show and well worth the watch.