I think the line is drawn by you.
Public Displays of Affection (PDAs) have both private and social issues. The private side is how much you and your partner are comfortable with. One hopes that he matches his girlfriend in PDA acceptability, and in your case, it's an issue.
The public side is more tricky. We kind of accept high schoolers and young college-types to be very affectionate in public. I guess we recognize that privacy is always an issue for the young so we politely look the other way and don't gawk at their smooching. But, we don't condone heavy petting or more in public.
It sounds like it's really an issue between you and her. It's not about what is acceptable in public unless she wants you to be an exhibitionist.
My parents are never affectionate in public, they don't even hold hands. I'm more comfortable with it for myself.
So Frank, don't ask us -- answer it for you and leave it at that!!