Sometimes it's just the right to preform their job in the same way that most other people in the exact same role can.
I don't blame anyone at all for preferring or requesting single-sex services, it's completely understandable, but then I would also have sympathy for a transperson who just wanted to do their job, but was rejected by an individual for no other reason than they were trans.
Ways and means to pre-empt this of course, and obviously it's no different a scenario to a female employee/practitioner/whatever being rejected by a male customer, or a male rejected by a female, but I'd still empathise with a transperson who found themselves in that situation because they are facing humiliation having done absolutely nothing wrong.
Up to the employer/workplace to ensure this shouldn't happen, but if someone possesses a GRC and is legally male/female, it isn't always possible because you can't disclose beforehand to random customers that the person they are going to encounter is trans.
I think it's a scenario that most people, male or female, could just shrug off. Been there myself, been rejected because the person wanted exclusively males assisting them. It doesn't faze me at all, but I'd imagine in a scenario where it is abundantly clear you are being rejected for no other reason than being trans, it must be quite jarring.