It's tough to argue with these people because you never know where they fall in their own ideology. Even in this case you have people who:

1 don't believe pedophilia exists in their ideology and will deny it as a right wing conspiracy etc... This person clearly seemed to be in that camp.

2 just use the "No true transman (Scotsman)" fallacy. Basically they say x y z doesn't happen and if it does then they aren't actually QT+. CNN basically just did this with the Colorado shooter after he said he was enby.

3 claim it's just an intellectual or academic exercise.

4 use postmodernism to deconstruct the concept of adult consent. The idea of informed consent goes hand in hand with this when they give hormones to minors.

5 are actually just fetishists of some kind and use queer theory as a means to arrive at the preconceived outcome.