It's honestly the exact same one that's always used in cases of homosexuality, "If we accept homosexuality, why not pedophilia?" I mean, it's kind of time we call this shit out for what it is. What he's saying, essentially, is if a person 'feels in their heart' they're 14 then they should be allowed to mingle with fourteen year olds and all that entails, and if we're not okay with that why are we okay with people changing their gender...because those two things are the same in some bullshit convoluted way.
I mean, it's all dressed up and hedged, but it's the same old insulting bullshit of equating in this case transgender with pedophilia.