My boys have always had semi-long hair, and from the age where their hair got too much in their eyes, also with bangs. They've been mistaken for girls SO MANY TIMES I can't keep track of it :-). I do not really MIND this mistake happening, to me and dh, they are obviously two boys, but I can see how people who are not used to boys with longer hair just mistake them for girls. I do surprise sometimes when people actually get it right :-).
My kids are also mixed so look a bit different than most locals, and the oldest has a finely built face for sure. I DO correct people about it, or (afterwards) joke with my child that the person was mistaken.
But what DOES offend is when people INSIST on calling them 'girl' or saying things like 'sister' or 'daughter', and definitely when they say sth like 'inappropriate hair for boys', mention the need for a haircut, etc. THAT, as other pp mentioned, is where offense does start.
I recently had a grandfather/neighbour suggesting (almost insisting
) on cutting MY ds's hair himself, he had a good razor at home (well how do you call that machine) and it would be done quick and easy if it depended on him. Also saying something that came down to 'normalising' my son by giving him a head shave as his grandkids always have (implying boys should have short hair). Just because I somehow mentioned I'd like to trim ds's hair a little (LITTLE!) but also that we like it this way and I just wait for a good moment to do that because he's VERY sensitive to things like hair cutting. His reaction was like 'bring him to my home and I'd do it for him' and 'just do it by force', 'the boy needs a haircut/shave'. Huh???
I know we won't socialise with that person again, like this time when we 'ran in' with him and his grandchild on the playground, no way!
He could have upset my son (spd-suspected) VERY much, and he did upset ME.
I also had someone remark 'I must have preferred daughters over sons' by realising they were boys, not girls.
KIDS HEAR that kind of stuff they are surely not deaf and insensitive to such comments! Sigh.
I have felt on few occasions that it WAS needed to reassure my (young) children that they were looking totally fine and gorgeous and that there's nothing wrong about boys with longer hair nor girls with shorter hair, but that it appears some people just have more gender-limiting ideas and get confused when sth is not according to what they're used to (in a way my kids understand). And I do randomly point out other boys with long hair to them as antidote to comments. So when someone seems to want to give us a feeling again of 'don't be so silly, just cut the hair', 'you womanise your son by keeping his hair long' or similar, I give an appropriate antidote to my sons if needed. We may mostly joke about the silliness of the person commenting, (note, only among ourselves, later and only when the person's comment was out of line or person was insisting)) about not knowing (my) boys are just beautiful with long hair (too) :-).
Also, if we would never had such (negative) comments, I would never have felt the need to 'explain' this to my children, but unfortunately, as shows the above, I needed to do this.