Let me say this very clearly: in public, your crotch, vagina, etc. is not your man's wubbie -- his, plaything. Your vagina is not a safety blanket or raggedy stuffed animal he can clutch while walking through a supermarket aisle.
I'm all for frisky public play, and rock n' roll naughtiness but keep it sporadic. That way its impact has...impact. If he wants to squeeze your butt once during a funeral -- OK. An out-of-the-blue foot-job for him during a group game of Jenga -- cool. But these things shouldn't be a habit.
Tell your man you get it, he likes you and that's great, but PDA has its limits. When your in public you prefer he hold your hands not your vagina. Or, if he wants to hold something even more personal how about your undying attention -
public crotch cuddling = a time out for him.