Interesting, I hardly ever have sex dreams... Since you're having them quite frequently, there's some thought that it might mean something. There's "pizza" dreams which are basically your brain processing the events of the day. So in your case, maybe you're having a lot of sex with your gf? and so its just getting processed by your brain in a different way maybe, or if you watch a lot of porn it could be the same kind of thing, your brain processing and filing away what has happened during the day.
Or... it could be that there's something on your mind troubling you that your brain is trying to solve. Maybe you secretly lust after other women, or are curious about women other than your gf...
or, maybe its just a reaction to hormones... I don't know how old you are, but some teenagers have sex dreams really often.
or, it could be something completely different, or maybe even no identifiable reason... On the occasions I have dreams involving sex, I usually have no way to identify what might have triggered it.