I mean, it's gross, but I can't support legislating sexual activity that doesn't harm anyone. Pregnancy is the grey area.
It DOES harm someone! Cousins? Sure, who cares? Even brother-sister? It's not hurting anyone if they don't reproduce. But parent-child? That is INHERENTLY without consent. I firmly believe that a child is incapable of giving sexual consent to his or her parent; they are inherently unequal, and that creates a huge problem. This goes WAY beyond thinking it's "icky," this opens up terrifying doors for kids who have "given consent to their parents" (the parents swear it's true, and so do the kids when they are in front of their parents!) and sick parents actually just manipulating their kids.
You are inherently "inferior" to your parent in this type of situation, no matter how ed up things are at your home.
EDIT: I am using the "general you" so the sentence flows better grammatically, I'm obviously not directing this at you. 
Yeah, I should have been a lot more specific in my phrasing here. Parent-child relationships are on a completely different level, as you explained. I think the mother and son in the OP are both legal consent age, and haven't seen each other in years. I do think, however, that parent-child relationships where the child is under the age of consent goes beyond incest and shouldn't be legal.
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