I agree with the above, but I also believe that the subject of a dream usually stems somewhat from reality... Are you perhaps slightly worried about the commitment? You're in your 20's, been together for a couple years, a good foundation that could lead to marriage. Perhaps you're a little freaked out about the idea that she could be the last woman you ever sleep with? Even so, that's a very natural thought, and it hits some people harder than others. But just because you're dreaming it doesn't mean you're going to cheat or even have the desire to do so. It could just be your mind's way to escape reality for just a few moments without your permission.