I'd call it a pathological personality disorder akin to a form of trauma adaptation. They are sublimating their full selves into a diminished identity based on a sexual fetish because jerking off is the only time they're happy.

People become singularly, instinctively focused on sex when they're horny. A trans person tries to stretch that sense of singlemindedness into an entire personality so they never have to leave that space. They obliterate themselves and try to replace their body with another sexual object so they can permanently detach from reality and observe it from the outside, like watching porn. That's why "validation" (lying) is so crucial to their identity. Being trans is one long extended edging session meant to stave off the inevitable return to self-awareness.