Why do boys enjoy sucking on their dads cocks?

There are several hypotheses. But I think that the main 3 would be:

1- Incest is allowed in the family, therefore boys like to suck;

2- The boy is molested but eventually he starts to like the sex between him and this dad;

3- Pretty much like the first 2 but the boy is gay to begin with.

But this is only my opinion...