Age difference suggestions?

I am 39 years old and my girlfriend is just 18. We have been together for about six months and things couldn't be better. We enjoy many of the same activities and genuinely enjoy spending time together. Our families, while initially disapproving about our age difference, have come to realize that we love each other very much and now get along quite well. Our sex life is full, vibrant and satisfying to us both. Now that she's recently graduated from high school, I intend to ask her to marry me within the next few months, and I expect that her answer will be yes. My problem is the way we're perceived in public. Anytime we hold hands or kiss in public people act like they're repulsed by what they see. It doesn't help that she's kind of smallish for her age and often wears her hair in braids or pigtails. If you saw her for the first time and thought she was 14 or 15, you could be forgiven. Does anyone have any good advice for how to deal with these people and their rude and nasty comments? We are both consenting adults where we live, are doing nothing wrong, and don't like having to explain ourselves to total strangers who act like what we are doing is their business.