I feel like if something is a dealbreaker for a person, especially something that's not observable, it's that person's responsibility to bring it up.

Here's an example of my own. A dealbreaker for me in a relationship is whether or not the person wants a big, extravagant wedding. I would not be comfortable in a long term relationship with somebody who was hell-bent on having a big extravagant wedding, because that's a lot of money, and money issues are one of the primary causes for trouble in a relationship.

But it's my responsibility to bring that up, and why, because it's my issue.

So, if you don't want to date a trans person? I'm sorry, but that's your issue, and therefore your responsibility to bring up.

I'm not going to get into the discussion of whether that does or doesn't make you a trans-phobe, the point is that if it's a dealbreaker for you, how are they supposed to know?

And of course, as other people have brought up, this ignores the point that me telling somebody that I don't want to have a big wedding isn't potentially going to literally get me attacked or killed. Somebody telling you they're trans might.