But equally certainly there are also many more gray area cases. Like people that do it because of more or less accurate stereotypes of risks involved in dealing with such people. Say, in case of apartments, its fairly well known, and in some areas probably very accurate too, that there is a disproportionate risk that the trans tenant to be is going to lead unusually vivacious lifestyle of some kind that will get the neighbors to complain or call the cops all the time.
If "unusually vivacious lifestyle" is a concern, "don't rent to anyone single and under the age of 30" is a much, much safer rule than worrying about trans people.
A lot of them have dress and grooming codes, sometimes quite strict, and sometimes for a good reason. They consider it absolutely unacceptable to look like a freak. Many won't even allow you to look like a perfectly average working class t-shirt+jeans dude. Nevermind, say, a member of metalhead subculture walking around the office and dealing with customers with unkempt long hair, three day stubble, wearing an unzipped black leather jacket presenting some suitably edgy band shirt.
Why is that so? These workplaces are very much insistent on "professional" corporate look, yadda yadda, and especially about not making customers and coworkers uncomfortable by workers looking like freaks, such rules sometimes being strict to quite extreme and hard to justify degrees.
Curiously, I have no problem complying with employer dress codes, and neither does any other transgender person I know. The people I know who do the crazy punk looks are all cisgender, and even they are perfectly willing to take it off when they're at work.
Because, you know, changing clothes is something people do.
On the other hand, the T lobby argues that there should be an exception to the "no looking like a freak" rules (formal or informal) for their social group, so that even the worst cases of "IT'S MA'AM!" should be immune from this kind of scrutiny, regardless of the practical effect they threaten to have on perceptions of the business, likely to far worse degree than the abovementioned metalhead.
You are generalizing trans people in a wildly inaccurate fashion here. There is no universal "T lobby" in the first place, and "Trans people should be exempt from dress codes" is not a position that is at all widely advocated, much less "every trans person wants this", and even more so this clearly biased speculation that trans people *might* be worse than metalheads.
For the record, I've told my employer I'm trans. All that means is I follow the dress code for female employees, not male employees.