Edge, I'm getting this more and more too.
For me, I say "Yes, I am his/her parent," in response to that. Deflects most inquiries. I introduce myself as "Suzi, R____'s parent."
I haven't got a lot of intrusive stuff, but if someone asked whether I was the mother, I would probably answer something like, "Yes, I am one of her female parents." Most people would just assume we're lesbians raising a kid together.