That's happened to me a time or two: someone thinks I just haven't explored all the options. When it happens, I'm polite and I listen. I figure you never know where a great tip will come from. (Case in point: I just heard about a local GT school that I'd never heard of before. Granted it's for 7-12th graders, but still...it was new to me.) But I do not EVER feel the need to defend my choice to homeschool.
I usually just say, "Thanks, but we looked at other options. Given where he's at, we're pretty happy with homeschooling." Then I throw out my patented <shrug> to show that I'm not offended, but I'm not talking about it any more. Keep it short, you know?
I don't entertain the notion that some casual stranger knows better than I do--not even for a second. Nor do I feel any need to defend our choices. I listen because I might learn something that could help someone I know if not our family (probably not our family! I've done my research...). But I don't get into it with them. A polite head nod and thanks, a shrug, and then we move on.
Don't let 'em get you down, LMom!