Focus on your boyfreind, enjoy your time with him, the company, conversation, and companionship he's offering. Who cares what other people are doing? Why is their attention more important to you then enjoying yourself with your partner? =S
Guys are visual creatures and they look...just like girls look. It's normal.
Honestly, I don't really notice other people. I just tend to focus on who I'm with when I'm out.
I've known some guys who did that for fun. If you look back at them and seems like flattered or attracted even when you're with your boyfriend, it will boost their ego as in "look, I could even attract a girl who already had a boyfriend lol!" So basically you've just become their rab lat for testing their power in attracting women.