In my opinion. A man strays because he can, he is allowed to, or because he is un-happy in his marriage and instead of doing the right thing, walk first then do what he wants, he cheats.
Because you know that he is doing this, the situation has to be that you are allowing him.
I understand that your thoughts are he sleeps with you every night, that is how you justify what he is doing, in that knowing, that means he loves you right? I get the feeling that you are NOT ok with it really, you are justifying.
If you were really OK with it, there would be no threads on the subject matter here.
Relationships take work, just like anything in life.. When I said "own it" I am talking about intimacy, between the pair of you, freezing her out. Saying NO... as in no more to him, sleeping with her. Own it..