Why is someone implying that you resemble the opposite sex and/or gender in 'manner and/or looks' one of the ultimate insults?

Most of you know my views on gender and sexuality – these sorts of views are not generally the norm and so here, with this question, I want to address people who are completely fine with the gender identity that they have/live in…why are insults insinuating that you ‘look like a man’ (if you’re a woman) or vice versa so especially upsetting? why the animosity towards the opposite sex? or is it animosity? if we put away the situations when homophobia and sexism are at play (hell, I suppose that took away a lot of situations, sigh), what are the reasons for being upset if someone out there tells you such things? is it because your specific gender identity is something you are proud of and don’t want it to be thought of as any other one? is it because you find the opposite gender identity to be so ‘alien’ and wouldnt’ want to be ‘one of them’? this is an interesting question, for me, because I know men and women look to each other for love (if you’re hetero) and the such and I know there’s the whole ‘battle of the sexes’ hoopla…but what are other reasons…does it have to do with what sexual signals we want to send out and the kind of mate we want to attract and keep? if someone thinks you look feminine (whatever that may mean to them) and you are a man, is that a problem, for you? help me out here…this also goes along with people being so quick to apologize if they think my children are girls but they’re indeed categorized as males…for some reason I don’t get why there’s such a need to let me know that they now understand their mistake…to me it makes no difference if they think they look like girls, so what…