My DIL and niece-in-law were just complaining about people calling their boys little girls this past week when they were here. They are pretty boys (1 and 2 years old) but I would not think anyone would mistake them for girls. I think some people have pre-conceived ideas -- maybe because they have one or the other themselves. I know I had a hard time gettiung used to calling Ben a boy as I had always had female dogs before him. Now that I have both Ben and Rocky, poor Candy gets called one of the boys too. I am much worse with names. When all 3 of my sons are here, I never get their names right first go around. They think this is just part of their crazy mother.
So, what I am beating around the bush about is -- don't take it personal! You know you have an adorable girl (or boy -- depending on who is reading this). Enjoy them -- that goes for pets or human kids! Anyone calling them the wrong sex is probably hitting themself upside the head and muttering "doh!" when they think of what they did.
Everyone calls my Ben a girl because I keep his hair floor length and he is "pretty." I just laugh and say "I know he is pretty enough to be a girl, but he is all boy and ornery as can be!" When I still had Sadie, everyone assumed she was the male as she had shorter hair and was larger than Ben. I guess it is just a natural assumption because most people thought that. Again, I did not get offended, even when she had something pink on. I just would explain that she was the girl, just a big girl -- like me