This.
He was punching before.
He's punching way more now.
He's old enough to be your Dad and middle aged.
You're only 25. You can meet your own - not second hand - man for a relationship and kids.
Even if he leaves his wife, he's washed up - 50% of his money, assets, pension will probably go to her. He's got kids to pay for until they're 18/21, and even after that; decent parents never really stop paying. Uni fees, masters fees, car help, deposit help,, crises etc etc.
That's all money taken away from you and any kids you had with him. That won't be the case if you have a household and kids with a man who hasn't had kids before. Who hasn't split half his earnings and assets with an ex wife.
Also you'd have to be a step parent to kids who'll probably resent you. That's not pleasant.
He's a shit deal
You'd be doing yourself down to get with him.
A couple of other points - he's a proven, repeated cheater .... You may have the advantage in age and he may have been "into you" but how long will that last when he actually has you. And you're not exciting and illicit and the "stolen moments" person anymore.
Maybe he'll be too old to cheat more, or maybe he won't.
(He's got plenty of experience at getting cheating partners, right).
Maybe investing your life and eventually kids in a not proven cheater would be a better idea (?)
Other thing; don't tell a potential boyfriend/partner about your affair or your thoughts about this man. That would not encourage most men to get serious about you. Quite the opposite. You were a teenager with a teenage brain back then, you've matured a bit now. Put it in the past and don't be so silly again.
Men his age pay through the nose to shag "escorts" your age, You were giving that for free, for some attention and flattery.
And he was controlling/trying to control you meeting anyone else - while having two women himself!
Nah; you can get your own man, you're worth that - one who's not washed up, old, hasn't got a load of baggage, hasn't done all the firsts with someone else - marriage, house, kids, and who cheated repeatedly on the person he did them with.
You can do better.
If you're not meeting men, you need to expand your hobbies, activities, social life and keep doing it until you meet someone. Think about the type of guy you'd like to meet and what they'd be doing/where they'd be going.
You are at the youthful prime of your life, with some people still single - you can do better for yourself.