Funny thing about marriage vows. To have and to hold, forsaking all others. It does sound like a promise of sexual faithfulness, but really you can't promise that you will remain wanting to have sex with this person for the rest of your life. Moral imperatives notwithstanding. It might be that your marriage is successful and you never tire of each other, and if things get a little slow, both of you working together can revive it, if you both really want that.
But if one of you does not feel that way anymore, no amount of morality can force that upon you. At least not in recent decades. Humans are entitled to decent sex if nothing else, at least that much. So Wife, you go girl. But you know, you really need to get your head on straight about it. You can't stand the thought of Husband fucking another woman? You do understand how crazy that is, don't you?
What are humans entitled to?
Not good health
Not decent job
Not freedom from political persecution
I say, at least let them have decent sex. As long as they are able to have that much.