maybe he's just being a guy
he wants more, doesn't know if he's ready
and when he said "when we are married---" realised the impact that would have, and tried to take it back. maybe he didn't want to tell you the extent of how he feels about you in case he scared you off
as for the sexual part, perhaps he really enjoys what you have now and is afraid that moving to the next level may effect it the relationship in a bad way -- probably afraid he will lose what you have now