First, in my personal opinion it isn't cheating if there isn't control over the dream.

On to the more...detailed...issue. Sex is almost never really about sex, unless it's wet...and even then it might not be. More often, it's simply about wanting connection with someone. And typically that someone is actually a part of your psyche. I'd be willing to bet these girls are exhibiting some traits that you relate to in your feminine side. i'd also be willing to put money on an animal having some role in the scene, if not directly related to the sex (no, I'm not saying you're into doing animals, just that cross gender references usually include animals regardless of the scene).

It might also be that you want more connection with your girlfriend, and you observe that there is more connection/communication with other girls you know - and none of that has to be sexual at all irl.