I get that all the time at work. Usually it's some smartass asking why the girl is pushing the heavy box while the guys are pushing the smaller boxes. I have a variety of responses: "because the work gets done when a girl does it"; "because I can"; "I guess the men just can't keep up"; "I'm pushing it just fine without help".

Once, a female co-worker tried to help lift something heavy that took several people, and she was elbowed out of the way by the guys with some comment about men doing the lifting. She responded with "Oh, I'm sorry, you're right. I better go before I get estrogen all over it!" I plan to use that the next time I get muscled out of the way for something I am capable of doing myself.