So banning incest is in their words discrimination against adult, consenting people, no matter if they planned to have a kid or not.
That's the reason why in Spain, France, Belgium, and a lot more countries, incest is illegal only if one of the partner is over 18yo and the other under 18yo. If they're both under 18yo, it depend of the situation, and if they both are over 18yo, then it's either legal or a rape, depending if it's consensual or not.
In other terms, what happen in the bedroom between consenting adult rest on the bedroom. Italy depicted it well in his law, since incest between consenting adult is illegal
only if it provokes public scandal.
As for the USA, there's some really weird case like Maryland, where incest is illegal only in case of vaginal intercourse...