If he wanted to sleep with other women, he should have stayed single. You are nuts to even entertain the idea of giving him your blessing to sleep with someone else. How can he claim to love you if in the next breath he wants to sleep with someone else. (But honey, I just need to do this. After all, I've been faithful all these years. Don't I deserve a little sex on the side. You know, just to see what it is like?)
I've been married for over 30 years. I've never slept with anyone other than the woman I am married to and have never considered sleeping with another. Why? BECAUSE I LOVE MY WIFE ABOVE ANYTHING ELSE.
But hey, do whatever you want. It is your lives you are screwing up.