My girlfriend says she "used" to be a lesbian?

Ok so here's the deal. I have this girlfriend, who i am madly in love with, but she had a lesbian relationship for over 3 years. She says she isn't a lesbian anymore, and that it was a one time thing. But three years is too long for just a "phase", right? And on top of that she has "hooked up" (had sexual relations) with more girls than i have. A lot more. And i catch her looking at the same girls i do, never guys. But when i bring it up- subtly, with love, of course- she says she is just looking at how the girl is dressed. Now she is talking about marriage, and this is literally the only thing i have ANY reservations about. Guys getting married and then "discovering" that they're gay isn't unheard of. I just don't want this to happen to me. I know she loves me, but is there a chance that she really is a lesbian in denial? or am I just being an insecure asshole? Any advice from similar situations would be great.