Sexologist here
https://www.youtube.com/watch?v=p-A8GvUehq4Watch this guy, he'll explain quite a bit.
So Let me put the basics of nature/nurture down for you, you biology probably has some sort of slider for potential deviancy, there is no gay gene but there probably are a series of genes that will allow for variation from the heteronormality I'd bet most of these genes are likely risk taking, indulgence and novelty seeking traits built into human neurology that are likely genetic.
The nurture is partially how you were raised by parents family church school and society but mostly based on your earliest experiences with sexuality.
Good examples of nurture have historically been chicken and egg but we have seen since the internet age that it's more about neural development about previous experiences. It's weird to equate the two but the reason that annoying catchy song get's stuck in your head and you learn to like it or can't help but replay it and the reason that kid that got ass fucked by a priest is gay now are the same principal of neurology, experience plus time breeds fondness in other words previous exposure makes the establishment of future neural pathing easier.
This is why kids who get molested go on to either molest other kids through their lives or have a sexual attraction to the people who are similar in appearance and personality to their abuser.
What the video talks about is mostly novelty drive inherent to the human condition expressed through porn addiction but also the triggering factor for male sexuality includes the penis because guys often have a Pavlovian response to their own erections they sympathize with another male's erection or in OPs case a traps erection but because of the female aesthetics he can mark the typical forms of female arousal too.
Essentially passing traps create a hyper-sexual response because they project male internalized sexuality and externalized desires for female sexuality.
I could continue at length by explaining my sexual encounters but I think I've made myself pretty clear.
Another emerging reason that needs research for male romantic interest in traps and shemale porn is the male mindset, taboo is an obvious factor in arousal, although often over stated but the fact that chicks with dicks were conditioned as male through their formative years lends to the belief that they have "reasonable male psychology" as opposed to the presumed irrationality of female psychology, essentially the mind of the trans girl is less alien and therefore less intimidating but another factor of brokenness and passivity is attractive to men and women today in the west are seen as too aggressive and over compensatingly masculine by many young men. It's almost paradoxical but the psychology of trans girls is less imposing to the men who fantasize about them, especially romantically, because it's a more feminine, less alien form of "female" psychology.