I think sometimes, for some people, when you're thinking of getting married your subconscious likes to toy with te idea of being with other people, either to make sure you are sure you want to get married, or just as a kind of way to fool around with someone else without actually doing it in real life.

It doesn't have to meen that you're holding deep feelings, that you didn't know about, for someone else in particular - you might dream about a different person another time, or you may not have the dream again.

This is all assuming you don't actually fancy this other person.