This should be fucking obvious but an argument I just went through on Twitter (gods I really should just not get on twitter) tells me that even left-leaning people for some reason sometimes think they are owed this information simply because they are interested in flirting.
Transpeople have no obligation to out themselves. even if you have or are going to kiss, or date them.
To think otherwise is to assume you are owed knowledge about their body. To get upset after they do tell you is to assume you were entitled to continue the relationship, or that they were trying to manipulate you.
If you have hangups that would prevent a relationship it's on YOU to be upfront about them or to accept the potential disappointment with grace.
I understand and that's totally valid - it's just that everyone gets emotionally invested on a different timeline, especially when their safety could be put at risk. I think deciding when to disclose that information should be done on a case-by-case basis.