Dude, I don't know. That's pretty damn beta. I mean, yeah, it'll look bad on you, but almost every guy (and girl) in the world would understand beating up a perverted creep for loveually harassing your fiancee. That's all he is. A perverted creep, a home wrecker, general scum tier. Yeah, he's roostery and he thinks he's smart, many people do.
You're giving him waaay too much credit IMO. If you can't intimidate him, you don't have to put him in a coma to get a message across. One well placed punch to the solar plexus and it's done.
Or, you can ignore it. That won't look very good on you, and I can almost guarantee it'll leave a pretty stuffty impression on your fiancee. I guess if you wanna be the merciful beta you can, but if you're confident your relationship will survive it then w/e. Maybe he'll get bored and chase after someone else.