Classical conditioning. I'm sure most of you are familiar with Ivan Pavlov's dog so I'm going to base this explanation on that assumption. To recap, classical conditioning is where a previously neutral stimulus is paired with an unconditioned stimulus (ie. an innate reaction), to produce a conditioned response. In the context of coom, this is works as follows:
the unconditioned stimulus (which naturally triggers a response): the sight of nudity, the sight of an erotic context/setting
the unconditioned response (the automatic reaction to the U.S.): sexual arousal
the conditioned stimulus (initially neutral but becomes associated with the U.S.): the fetish, in this case being transvestitism
conditioned response (learned response to the C.S. after conditioning): sexual arousal
In the beginning of their porn addiction, they're not into the fetishes yet. In the beginning, they're reinforcing the nudity -> arousal pathway that's innately programmed into most normal humans. However, they start to pair that with masturbating and orgasms, and because it's the internet, they get bombarded with other, slightly stranger things that they wouldn't have looked up on their own, but because they're already aroused and their dopamine receptors have downregulated from consumption of the normal stuff, they click on it. They pleasure themselves to that new stimulus, and now that new stimulus (previously neutral, but now conditioned) has been associated with that original pathway. This repeats itself over and over.
Eventually one of three things ends up happening. Firstly, they watch videos where they can mostly only see the woman, and just see her being on the receiving end and they see her exaggerated displays of pleasure. The person who is masturbating to this is also feeling pleasure, so then he begins to associate her pleasure with his pleasure. As in, he self-inserts into the porno as the woman. Second, they run into tranny porn and find it erotic and confusing but can still get off to it because the tranny looks at least half a woman and so they again classically condition themselves to be aroused by that stimulus. Thirdly, they run into sissy hypno. This is a form of video where suggestions alluding to trooning out, or explicitly telling the viewer to troon out are played over porn clips.
Essentially: when you masturbate to something, you are conditioning yourself to like it. It's very similar to training a dog with treats. You begin to associate the pleasure and orgasm with whatever you were watching.
Have you ever enjoyed a certain food a lot but then one time it suddenly made you very sick after having it? Did you develop a strong aversion to that food after that incident? That is also an example of classical conditioning.