I will tell you of the situation I was just in. A friend of mine who has some issues with addiction, but who is a really good guitarist was asked to try out for my band, we tried it out because he has talent. Unfortunately, things didn't work out that well because he looked at the band as a competition not as 5 people working together to create something better than any of us could do alone. He didn't practice very hard to learn the parts we asked him to, but always was like hey guys I want to show you this rift I made up. He also expected us to pick him up and drive him home to and from practice. Otherwise, he would be late and leave early, and even though we told him before we offered him the position as rhythm guitarist, that each practice would cost each band mate $10 he never coughed up.
We kicked him out of the band. I tried to still be his friend, but he would get stoned and drunk and show up asking to come back, he acted really stupid and I lost a lot of respect for him. I always liked his girlfriend, but never let it show out of respect to him. I tried to explain to him that different aspects of life should be kept separate. The band and his friendship were two different things. I don't think he ever got it, because of how selfish and immature he is (his coke addiction has emotionally stunted him to the point that he still acts like he is 16, he didn't start doing coke until he was 18 but the whole time he hasn't ever grown up). Anyway, to make a long story shorter he cheated on his girlfriend, I consoled her (she is my girlfriend now) and I made him so mad that if he thought he stood a chance against me in a fist fight, I am sure he would have tried. I lost a former band mate, a friend and ultimately I am okay with it because the band is happy, she is happy, and I am happy. I hope one day he can come to terms with it, but I am not holding my breath.
What I am trying to say to you is: Stay in the band, accept that he is going to be the one all the girls lust after, be mature and honest with him about where you would like the band to go musically in the future, never again mention the girl but when you start touring and she finds out he has cheated on her, console her ... be there for her, tell her how bad he is, how good you are, that you would never do that to her and she will be yours. If he isn't the kind that would cheat on her, then you get to bang all the chicks on the road that want him but can't have him because he has a girlfriend. You don't get the girl of your dreams but you get a lot that will help ease the pain of losing her. Either way, it can be a win-win situation for you, if you are mature and start to look at the bigger picture. Cheers.