Who cares if she's fucking other dudes (oh, she is!)! Hang out and bang it out and have a good time. Stop projecting to the future and looking for wifey. Relax and have fun bro.
I'd bang her for awhile (wrap it, of course)...but she's not relationship material obviously. Let her believe you're in a relationship with her for a little while though, as long as you're having fun hitting it.