The contentious bit is whether it should be offered at all. If someone asks to have a healthy leg amputated that request will not be completed. The person will be referred for mental health support. Many would argue that surgical alteration of the body to support the concept of gender transition is no different.
The argument for the difference in treatment is usually about suicidal ideation, but I remain unconvinced that these operations should be offered. I'm still on the side of the fence that says we should be treating gender dysphoria as a mental illness rather than a natural biological state of being.
That said, I'm very much aware that being gay used to be treated as a mental illness whereas we now know it is a natural state of being. It requires no treatment however, which is the key difference. Who you are as you is great, whoever you are and whatever you like. Once it starts requiring surgery and medication, it isn't about you being you anymore. It's about you *not* being you IMHO.