As fucked up and weird as it is, they're both adults, and I don't see why they can't fuck each other if they want to. Yes it's fucked up, but they're both consenting adults, it's not like he's raped his teenage daughter or something. I think faggotry is pretty fucked up, and gays and lesbians can marry and fuck all they want, so why exactly should this be any different? All it is, is the currently accepted norm in society to stand against it.
innbreeeeeeeeeeeeeders!