I need to take another hiatus from this place but wanted to respond before I go.
Instinct, compulsion, undoing, mis-association of pain/pleasure, unbearable hurt, trying to take control of what was forced before, addiction, risk-seeking, proving we are bad, devaluing ourselves, many more possibilities and combinations thereof.
I am not married but have been in serious, committed and monogamous relationships. These are only a few of the reasons.
I can only say that it took me a long time to understand that it is nearly impossible to explain to an outsider and especially someone who is close to you.
I might recommend this - can you black box it? In other words, accept that it is not "ok," that it hurts you and that that you don't understand it? That sounds like it'd be very tough to do, but it might give you the most peace.
The only good news I can give you is this - I guarantee you that if he is getting good help and his recovery is proceeding, there will come a point when he will be able/willing/and determined to explore, confront and stop acting out (meaning all avoidant and self-destructive behaviors).
I wish you luck. It's a long road. Try to take care of yourself first and foremost.