That's pretty harsh, body image, regardless of gender, is a fairly touchy issue, and from someone who you're going out with you'd sort of expect a level of sensitivity and understanding (At least I would expect that, maybe others are different). It's really not something I'd be happy to put up with.
Personally I'd just point out to them that, even if their family think I'm overweight, I didn't want to hear about it, especially from them. I'd make it very clear that I'm offended by it, and point out that it's my body, and that at the end of the day, if I'm happy with it that's all that matters.
Failing that, I'd tell them to shut up or go back to having sex on their own. I'd hope that would make the point very clear.