Husband (M39) asked my (F38) permission to have an affair and I don't know how to react

Last week my husband asked if he could talk about something serious. I said yes, of course, we are always very open and honest with each other. He tells me that he has been very distracted lately looking at other women and he hopes it hasn't bothered me too much. I have noticed him staring at younger women lately which isn't normal for him but I figured there is no harm in looking. It's not a problem for me if he isn't obvious about it and I tell him so. He asks me to tell him if I notice it again so he can stop. I say that's not my job and tell him he can stop on his own. We keep talking until he blurts out that he is thinking a lot about what it might be like to have sex with someone else.
He knows I had several partners in high school and college. He spent all that time hanging with friends and playing video games. I was his first kiss. Nearly a year later we had sex. I was his first. We've been monogamous and faithful ever since. He promised that was fine with that when we committed. He promised again when we got married. Now nearly 15 years later he isn't fine any more.
He says he'd like to date and have a fling. No way. How is that fair to me or her. He offers to have a one night stand or hire an escort if it makes me more comfortable. How the **** is that more comfortable. I know that if he does this it won't stop after one person. I can't do an open marriage. I've been cheated on before I met him. I know how it feels. He doesn't have a clue. I'm crying constantly and I keep thinking this has to end in divorce. He offers a legal separation and says we can reconnect in a few months. It doesn't work like that. I say no. I ask if he really wants to divorce over this. He says he doesn't. He does not have to have this he just really wants it. He promises he will get therapy and learn to accept he is a one woman man until he dies. I'm crushed when he says this. I feel so petty. He assures me he can be fine again and asks me to just not worry about it. He tries to joke about really wanting a yacht but he will be fine without one as well. I try to laugh for him.
It's been a week. I'm still crying whenever I'm alone. I can't think about anything else. I can't sleep next to him. I lay awake until he falls asleep and sneak into the guest room. He hasn't mentioned it.
I don't know what to do other than file for a divorce that neither of us wants.
TL;DR: Husband has never slept with or even kissed anyone except for me. He wants my blessing to **** another woman before he turns 40. I'm angry, sad, confused. I love him to death but I think I need a divorce if he can't be faithful.


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