DC already has the meta-gene as an in universe explanation for where most of their earth born heroes come from. Although I haven’t been reading the New52 stuff, so I don’t know if they have continued to use it.
Well, her origin story does have her falling in love with a man, a crashed pilot she nurses back to health. But I don’t know if it has her being a lesbian before that. Seeing as she’s the youngest one there, it wouldn’t really be necessary. She’s just barely an adult, if I remember correctly. And a lot of Themyscirans are celibate.
It makes sense, since they are they are the embodied spirits of slain women. At least some of them are going to be exclusively heterosexual.