The fact that he likes it says a lot. It's wrong on all levels and for all this time he has been cheating on you, have you considered he told you that because he feels guilty that he's with you and doing it? Not about the fact that he's actually having sex with his mother.

Is this really a trait that you want a longer term partner to have?

Whether it is or isn't his fault, it is dangerous for you, and he is putting you in all sorts of danger.

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