JOhan, come on now -- you CAN stop yourself from having sex with 5 other girls, at any time, and especially while you're supposedly in a committed relationship with your gf. Do you think so very little of yourself as a man, that unless your gf is someone who all other men will turn to look at, you feel you have to prove something ?
Are you really so shallow that you want to spend your life with a bimbo, whose looks will fade in a few years, and who has the personality of a potted cactus and the intelligence of a jar of peanut butter ?
Frankly, I agree with lady nina, I don't think you have the faintest idea of what love is, and should do this decent woman the favour of gracefully withdrawing and allowing her the chance of loving and marrying a real man. She deserves someone far better than you, and you don't deserve anyone as good as her.
I absolutely don't buy the myth that someone like you "Can't" help themselves but screw around, howver much misery it causes to others. You can help it, but you don't choose to. You have problems with vanity and sound rather like the male equivalent of a bimbo ( ? a bimba ? ) so maybe you deserve the promiscuous dimwits you chose to spend scraps of time with.