You're being shown disrespect. Plain and simple. If you let it go, he'll accept your weakness and probably try something down the road a little more tempting. Hey, is this the end of the world? No. But guys who don't show proper relationship common sense during the dating phase aren't really the ones you call "keepers". Doesn't matter how hot she is, he's doing you and thinking about her from time to time, and that is reality, Sweetheart. You don't have a poster-length photo of your ex sporting his banana hammock at Padre Island few years back, now do you? Didn't think so. Get this guy on track, gain back some control over his cheat'n heart or find a guy who has your picture on the dresser table.