Mike, you sound like a doink who cares way too much what other people think. Are her looks the only thing you find attractive about her? A hot-looking partner is the most trivial thing to find out there. As you point out yourself, it is just so easy to do. Maybe your GF is thinking about more interesting things? Like finding a more interesting, less shallow BF?
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh