I read some survey years ago that said that men generally like women's hair to stay how it was when they met.
The theory is that even men who don't profess to have 'a type', will have strong residual attraction to the person's physical appearance when they were 'falling in love' in that early lust stage. For men who do 'have a type' its even stronger.
So if he likes blondes with long blonde hair, that's his type, and that's how you looked when you got together he will want you to stay that way.
If he's more relaxed but you had long blonde hair when you got together, he will get those same nostaglic lustful vibes associated with that time - a bit the same way that some aftershave can have the same effect on a woman - if it reminds you of the hot exbf who always wore it.
Women are the same but notice it less because men rarely have radical changes to their hair length or colour by choice. Beards or facial hair are a better comparator for women
It's no different from finding a man less attractive because he grows a beard or gets fat.
He's entitled not to like it. You are entitled to have your hair how you like. Just how he's entitled to grow a beard or shave it off and you are entitled not to like it.
Question is how strongly do both people feel about it and why ( for the one who doesn't like the change: genuine issue/affect on attraction or just general grumbling or control issueand and for the one who has made the change: whether they love it or could live with out it or whether they will feel upset/controlled) nad whether it's something you are prepared to compromise on or not.