I just finished writing a sex scene between an alien shape-shifter and an alien child from a planetary cycle different from Earth. The child is 11 years old on his planet, but their species can easily live 200 Earth years. Same story also features a rape scene with two androids that are technically under 5 years old, while their bodies are those of teenagers. I'm not entirely sure how old they're supposed to look, but since they're fucking robots that emerged fully grown from the tanks, they're not going to get any older no matter how long we wait. And while this story doesn't feature him, let's not forget the show's main character: 11-year old boy by day, 50-year old monkey man whose body looks to be around 25 by night!
And what about the 200 year old vampire that had sex with a 16 year-old girl in the source-material? Or the months old child of rape force-grown to match his mother's age? Newly born clone of a being older than the universe marrying the boyfriend of the woman the older-than-the-universe being copied? I mean, it's not like we'll come across any absurdities dealing with fiction, right?
But as long as we're clear that these things are illegal in the US, then okay. I'll stick to the of-age incest? Or maybe I'll write porn featuring animated cotton swaps, because I've wanted to write that for a while. Not minors!