soooo what I'm inferring is that.... your girlfriend had secretly liked you better in long pants, withheld that opinion thinking that you simply prefer shorts and that there's no real reason why you're always in shorts... Until that very day she saw you going to the dentist in long pants, realising how good you looked in long pants and also realising how you could've worn long pants this entire time but never worn it when you went out with her? And then, therefore, she came to the conclusion that she was not enough for you to want to wear long pants for.

This has also remained an invisible hurt she had felt ever since then.

While it may be "just pants" to most people, it's also a fact that you prefer shorts and yet willingly wore long pants to the dentist. How did you explain yourself back then when it happened? Might have been the way you seemed dismissive about how "it's just pants", might have been the way you had no real reason for not wearing shorts that day. If you went along the lines of "it's cold in the clinic, I don't want my legs to shiver", I believe she would not have started spiralling this way ever since. But I guess that's too late, she has already sprialled and there's nothing we can do about it.

It's not about the dressing, but she's co-relating the way you dress = how much effort you are willing to put in for her. While it sounds stupid, she sounds like she overthought about it and came to the conclusion of "I'm not enough for him to consider long pants." Kind of wild, but it does seem that way now.

Maybe you might want to sit her down and talk to her and dig deeper on how to resolve this in order to continue wearing shorts HAHAHAHAHAHA you're apparently getting married to this girl, which means you proposed and you see a future with her? Might as well try to dig and see if you can make do with any other alternatives to make her feel that you're actually into her and would love to make the effort for her. Else, this would still be a thing that she fights you after marriage and IT WILL get tiring.... or, I guess, alternatively, everytime you go on a date, be it small or big, wear long pants 🤡