There are so many people with hereditary genetic disorders that we allow to reproduce. We also allow people to reproduce that have absolutely no means to even take care of a child. My main point is it shouldn't be a criminal offense when two adults want to have sex. Also abstinence is not the only way to 100% ensure there won't be any children. Option 1 birth control, option 2 condoms, option 3 morning after pill, if all else fails option 4 abortion.
The methods you point out are, as I said, not foolproof. Pills, condoms, the 'loop', they can - and eventually will - fail. Statistics, happens all the time. And as I said, as long as there are more than one person involved, you cannot be sure what the other wants and does.
Plus:A relationship breaks up, in so many cases. You walk away, and just try to blend that person out.
How do you do that when your former lover is family? What do the others do, mother, father, did you think about them?
Incest is about SO much more than just sex