Yes it does suck on your end but on the other side of the phone your perspective date is probably having a whole mental breakdown about it. For a lot of trans folk disclosure is absolutely nessisary as early as possible and preferably for safety reasons not when you are face to face...

Buuuut they also are very likely to get really vile transphobic backlash from a perspective date as much as they are honest rejections based on genital preference which sucks to be rejected for but is nobody's fault. There's a lot of trans people out there who feel like they are never going to be given a chance. Either way steeling themselves for one form of rejection or a vile reminder of the awful people out there who think you are subhuman and are offered up a nice juicy target on which to let loose their bigotry does tend to make for disordered social niceties. Once someone has been burned enough they get pretty damn shy and the procrastination is more of a case of battling personal traumas until the last possible second where one absolutely must do the right thing.

I would advise not taking it too personally.