Im dating my sons girlfriends father
She may be terrific with you, but problematic for her mother.The mother may be worried about sexual activity, but she may simply be trying to regulate her daughter's behavior (or backtalk or grades or whatever) by controlling the most important thing to her daughter - your son.Pick up some books about self esteem and making good decisions.
The one thing you have to realize though is that you don't see the same side of your son's girlfriend as her mom does..don't live with you.
You have to try to support a parent's right to make decisions even if others may disagree with them.
I understand that you worry about rebelling and getting into unhealthy choices/decisions, and I agree that excessive control can do that. And maybe that's what this mother is responding to - a perception (however incorrect) that you are too permissive, or that other parents in town are too permissive.
C may always act that way with her daughter and her son-in-law, and then shake your head and leave it at that.
You might be surprised how smart your son will get.I would strongly advise you NOT to engage in discussions with the girl about her mother, and to be very careful what you say to your son.