First, of course, as soon as you see your first "regular" porn (though in my case it was some German stuff, so it was quite... powerful), you immediately "get it": "That's the s...it". But then I guess it's natural to require more and more stimulation (if you don't do dopamine fasting or whatever you might call it when you just lay off the stimulant, in this case porn, for a while), so you get, for example, from blowjobs to deep-throating, then to irrumatio, then to vomit etc.
What's interesting for me are certain kinds of porn where the "gateway" is not that obvious (though for zealously religious etc. people even oral sex is off limits). I mean, let's review: kissing is beautiful, why not kiss the girl's "flower"? Or why not ask her kiss your thing? And then lick it, and then suck it, and then take it deep... deeper... and so it goes
But what about bestiality for example? I remember how I had a gut feeling that I shouldn't watch it when I first watched it out of curiosity. But then, as I returned to it over the years, I slowly got used to it (or is it that the "man-made limitations were broken" and are we all actually pan-sexual outside the culture?).
Same with she-males. Now I'm not even that appalled by gay stuff when I happen to see it (and that's another thing: I see it as another zeitgeist thing that we're so much more OK with lesbian stuff than gay stuff), though it still does not attract me if there are no girls there (which is a whole trope in the female domination industry, of course). The guys going at it just seem too enthusiastic about it. Maybe it's the same reason why I don't get why people are so enthusiastic about guns, or coffee, or jogging
And then comes scat. Maybe I'm just as oblivious as I was when I first saw bestiality (because, hey, you get used to anything, and those two girls with a cup only shock you the first time), but I think that even if I watch it again and again, there is just nothing there that hits "that something in me". With bestiality, she-males, and even gay stuff, it's easy to draw the line: it's all about sex and sex organs, and they're doing what they're made for, more or less, sexually
One could argue that scat is also deeply psychologically connected to anal and humans' psyche development (Freud etc.), but it's just not that obvious. But I think it all just comes down to what we are wired to find sexy — though, at the same time, I cannot deny that the environment, experience etc. are also significant influences)
What I wanted to say in short is...