As 'slut' is a derogatory term for a woman who sleeps around, then calling someone a 'slut' because she wears short skirts is NOT an opinion - it's just a totally false and meaningless accusation. The length of someone's skirt has nothing to do with what she does in or out of the bedroom - you can't possibly know what anybody does privately based on their dress sense.
Saying you don't think short skirts look nice on women at 40 is an opinion (one I disagree with, for sure, but still, that would be an actual opinion and not just rudeness and aggression).
Secondly, 'slut' is a misogynist term designed to penalise women for their sexual behaviour - which is, after all, their business. Many people would say that there is actually no such thing as a 'slut' and that anybody who thinks there is is beyond sexist.
For the record, I never ever said you were immature or clueless and nor would I - I don't know you. Nor did I jump on you - I was polite and made my point carefully and without being personal or confrontational - so I don't know why you got defensive with me.
But swearing, sexism, aggression, rudeness, ageism, and prejudice are never going to get you very far.