Dude. Relax. What your talking about is actually a really common fantasy. Sure some people would see it as odd, even crazy, but those people might be in to feet as a fetish and to me that's weird.
What your talking about is such a common one it's even got a name and such a big following it appears on TV documentaries. It 's actually growing in popularity faster than any other fetish according to this poll I heard about. Don't know if that's true.
It's called cuckoldry or cuckolding.
Guys who get of on the idea of watching their girl cheat on them are cuckods. The girl/wife a cuckoldress or hotwife.
It spans a great number of other fetishes sometimes, but is usually a form of submissive role play.
Does it turn you on to think maybe she likes him more, that she might keep you as a play thing to lick her out but reserve f***ing for the other guy. Does it turn you on to think of her saying he's better than you or bigger than you or that she'd do things for him she'd never do for you? Like anal or threesomes?
You're not a freak or weird. You're actually quite run of the mill as far as kinks go.
Now the only thing to decide is... if it turns you on a lot...but you'd never want her to actually leave you, but fancy her cheating in front of you, you should really consider suggesting an open relationship with her. You get to keep her, you get to watch her.
But be warned it can be a tough sell to a girl surprisingly enough. Ask a guy to cheat-easy, ask a girl to cheat and yet stay with you... hard, but not impossible.
If you like the ideas, check out the following: link link
AND most important, if you want advice on becoming a cuckold, try this: link