To (kind of, since I don't remember exactly how he phrased it) quote a comedian:
"Having a girlfriend makes you feel guilty about some things. Like, before I'd see a hot girl on the street, and I can't help but picture myself, y'know, having sex with her or whatever. But now that I have a girlfriend it's like I'm mentally cheating! So, what I do now is, I'll see a hot girl, and instead of fantasizing about being with her, I imagine myself having sex with my girlfriend... AND the hot girl. I mean, she's gotta be ok with it, she's RIGHT THERE. Look what she's doing!"