Her cheating may have bought his submissive side out and he’s strong enough to acknowledge that now and act upon it.
Someones mentioned control but in a different way to how I see it. Having his trust broken will take a long time if ever to overcome, so for him he’d rather be in a position where he has some control over what may happen mostly in his mind. If he has an erectile dysfunction or a small penis (or he thinks it’s small) and it was or he thinks it was the reason his ex cheated then he may really want to control the situation. My partner was not cheated on but fixated on his penis size which is smaller, at first I dismissed his suggestions but he persisted and after a time in all relationships sex can become a bit routine. I want to add that sex and intimacy can be separated, I value our love and keep the intimacy strictly with him. So over time, the routine and his persistence I began to open up to the idea, we communicate really well, online we found and read a lot of legitimate educational information which up to that point I’d never heard of hotwifing and I have since learned that I am a Vixen and my partner is a Stag. It was still some time till it happened and I’m not giving advice just sharing our journey. We role played a lot, with sex toys, dressing up, meeting at motels, he even wore those sheaths and surprised me as a well hung lover. As I said earlier he was fixated with size and he wanted to share and watch me with bigger, I was more interested in a guys skills but those sheaths wet my appetite and I confess to now being a bit of a size queen which my partner tries to overcome with the most amazing and passionate sex after I’ve been with another man.