Regarding incest: I guess if both parties are consenting adults, as much as it pains me to say this, however they get frisky in their own time is none of my business. Deformities in an incestuous lovechild are a pretty clear indication that it's probably not the best idea but, y'know, none of my business as long as it doesn't affect me.
You can't compare homosexuality to incest because one is a conscious decision (well, I hope so anyway. I guess it's rape otherwise) and one is not.