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.
To make this extra fucking clear, Finding out someone's body isn't what you expected does not mean you have been tricked. It should not be abnormal to find out unique things about a person. finding out someone is trans should be no different than finding out that they have a glass eye, or they are double-jointed, or they are deaf in one ear, or any number of other conditions.
Normalize trans people, don't categorize our bodies as "special" or "other".