My parents say that I'm not allowed to wear women's underwear because I'm a guy.
Do you think your parents just want to make sure you act like the gender you were born with?
Yes, but they also act like assholes and tell my girlfriend that I wear women's underwear.
You are an adult so you can technically now wear whatever you want as long as it makes you feel like yourself. It doesn't matter what other people say because at the end of the day it is your life. You might create mental health issues by doing whatever other people want you to do.
I know, but it's just my family are being real assholes about the whole thing.
I am glad that you feel comfortable enough with yourself to recognize that it is okay to be yourself and your family's reactions have nothing to do with you. Just try not to insult them for doing what they think is right.