i was in a sort of similar situation a while ago. i mean, it's not as if the ultimate goal for every relationship is marriage, but if you know already that it's never going to go anywhere, why prolong the inevitable?
i hear ya. i've had smiliar things happen to me. i was happily living with my husband (we weren't yet married at the time), and we were expecting a baby. one night, i dreamt about my first love, who had left me while on a drug binge a few years earlier. it made me wonder how he was, and what he was doing. after that, i would start thinking "wow, what if he was still here" and all of that, and it made me question what i was doing with my soon-tob-be husband. then, out of the clear blue sky, the ex called me. and after that, it made me realize that even thoug hhe'll always have a special place inb my heart, and i will always hope that things work out for him, that i was thinking about him so much because i had no closure.
sorry for the long story, and i'm not sure if that's what you're looking for. but in my experience, dreaming and thinking about guys past happens because there's still a little bit of that "what if". maybe its happening to you right now because you're not sure of the current situation with your new guy. good luck!