So some backstory. My girlfriend has really beautiful curly hair. I know it's important to her and I love it. When we went out for New Year's, she had it straightened by her friend and it looked incredible. She's never straightened it herself so I was surprised when I saw it, but in a good way. She looked great. She was fielding compliments all night from our friends who have never seen her hair straight before. At one point she mentioned that it was a one-time thing given how long it took (3 hours) and that she kind of missed her curls. 

Fast forward a week and my birthday is coming up. She sent me a few pictures of dresses and asked me which one I wanted her to wear. She does this sometimes when she wants to look sexy for me in particular so I was all for it. At one point I asked her if she would consider straightening her hair because I thought it would look great when we go out to eat. Out of nowhere she got really upset and offended. She told me people have told her to try straightening her hair her entire life and that it makes her feel like her curly hair is inferior. I didn't know this so I apologized and asked why she decided to do it on New Year's. She replied that she just "felt like it" and then asked me if I would be more attracted to her with straight hair. Of course I said no, but she was still upset. Most of the time she wears it in a bun on the top of her head anyway because of how much work it takes to maintain. 

Apparently she got teased a lot growing up because of her hair and it took a long time to get confident in her natural curls. I expressed that I find her hair completely beautiful and the straight hair was only a suggestion. She took it to mean that in order for her to look extra sexy on my birthday that she would need different hair. At this point I just kept reiterating that I love her hair but she isn't over it. She told me that it would be like her telling me to wear lifts in my shoes to look taller. I can't change my height and she can't change her hair which i get. But I never asked her to change it forever. Am I the asshole?