Exposing your genitals in public is a misdemeanor offense. Sexual contact with someone under 16 is a serious felony offense. It is never a good idea to have sex in any place that can reasonably be construed as a public place.
I was a public defender for 12 years and now I'm a private attorney. I was a good lawyer then and I'm a good lawyer now. My point is there are good public defenders and bad public defenders and there are good private lawyers and bad private lawyers. Your situation sounds pretty serious and you should have a lawyer you feel confident with. You should probably interview a couple lawyers and get a feel for price and ability and then see if you qualify for a PD and analyze that lawyer the same way....
The seriousness of this depends on whether it's being prosecuted as a criminal charge or as a municipal ordinance violation. If it's criminal, you should have a lawyer and you may qualify for a public defender, depending on your income. Either way, you should try to find out if there is a deferred prosecution program you could take advantage of to avoid a conviction.
If the girl tells that exact story to the police and prosecutor it's extremely unlikely there would be a prosecution. Theoretically, you can withdraw consent, but if a girl let's it go that far before she withdraws consent, most prosecutors would not think that case would have prosecutive merit.