I'm trans. I think the reason people get upset is because of the reason behind the preference. That reason can be totally valid, to totally shitty, and people assume either the best or the worst depending on their perspective usually. A trans person seeing this might assume "oh boy another transphobe" while a cis person would think "yep and I won't date a person who owns cats because I'm deadly allergic" and carry on.
On top of that a blanket "I won't date trans people" rule tends to ignore "outliers" like nonbinary, intersex, or gender fluid people, which can feel pretty bad when the underlying reason doesn't fit the actual real life scenario you might be in.
Here's a few reasons behind the preference that I can think of:
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I'm straight and want bio kids - Fair enough, either your partner is cis or they aren't compatible for that goal, or you're getting into the nonbinary grey area.
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I don't want to deal with the extra baggage, potential judgement from peers, or mental/physical health comorbidities that come with transition - kinda bleh but its your choice and only you can really decide what you're willing to invite on yourself, but it feels crappy for trans people who definitely didn't ask to be trans either.
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I'm attracted to specific genital configurations - that's your preference, but it sort of ignores any post op trans people, which feels really shitty if as a trans person you've done this major surgery to be happier and more yourself, and people just assume you haven't, and won't talk to or engage with you over something you already changed, through a very intensive and difficult process. Overall though genital preferences are perfectly fine, people just don't like saying "I only like penis" because it sounds weird, so they substitute "I only like cis men" thinking that's a less awkward alternative.
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I don't think trans people are who they identify as - real shit and the kind of people most are actually mad at when people day "I won't date trans people.
Anyways, I'm sure there are more, but the point I'm trying to make is, saying you won't date trans people is just kinda vague. Many people will assume the worst, which is on them, but it would help to clarify and be clear about what your real preference is, or why you have it. For instance "I want to have bio kids someday, so if we aren't compatible on that level then that is a dealbreaker" or "I'm straight and I'm only interested in Cis or post op trans women" which with a couple extra words clearly portrays that its a genital preference without explicitly saying it.
TLDR: having preferences is perfectly fine but when people judge you it's because it isn't clear why you have that preference, and usually ignores outliers. Clarification, while maybe a bit longer or more complicated, would eliminate the anger in most cases. The people who still get upset are likely to be angry no matter what.