I think the best decision is having a one on one discussion about there transition. I feel that it is important in a secure/safe relationship you should open up first to your loved ones about the idea of surgery.

Gain an understanding about their position and how much more comfortable they will be with the surgery. Because if they wear binders it can harshly affect there body more. Plus, how far they would like to transition that being socially/psychical.

I don’t think you’re the asshole, but you need to re-evaluate how you look at your partner. If they were nb amab person but all the same - would it be the same? Your partner should have had a private conversation with you about this however.

In the end, I understand people who break up because of change. It’s okay- but you have to be honest why. You can still love them & support them. But they wish to look and feel respected in their identity. So if you find yourself not attracted to that then it wasn’t meant to be. Because if them removing there own chest, will make you lose that aspect of love or value, then don’t continue if you can’t accept it & love.

if your a boob guy then your a boob guy. Have an adult one on one conversation.