Its natural to look atthe world around you and the people around you (I call it people watching and have great fun trying to figure out people just by looking at them LOL).
Do you really look at other guys or is it just something that he thinks you are doing? I'm not sure how you can stop looking at what is around you when you are in public places. Seems like he needs reassurance that you are not going to leave him, not now and not in the near future.
I have to say that eventually as you get older, the possessive behaviour that seems loving at the moment (as I see it, I could be very wrong) will eventually wear you down and you will just want to get out of the relationship. I hope that in this case I am wrong and he is just really scared of losing you... he needs to change his perception of what he thinks you are doing.. you are not staring at other guys, you are just looking at what is around you xxx