Amber does it exceptionally well...
It is a subject of great debate in the soft sciences why we want what we do in bed - is it hardwired or the product of formative experiences our conscious minds can no longer actively recall? (By this, I don't mean sexual orientation. If you need me to tell you that I think sexual orientation is part of your hardware, not software, you haven't been paying attention.)
The desires are there, no matter the provenance. The human animal becomes infuriatingly unpredictable when it comes to sexuality, as so many people have kinks and quirks one could not guess by looking. (I would imagine most everyone has a slight divergence from the supposed mainstream, be it for dental braces, men who look and act like Gregory House, or women who can sing. Any preference taken a shade too far becomes a fetish.)
Dan Savage, sex advice columnist, says that a sexual partner ought to be Good, Giving, and Game. That means that one should at least consider a partner's request to be tied up and called a naughty slut. One does not have to necessarily indulge their desire to be peed on in a threeway while you eat a hearty salmon dinner, but it couldn't hurt to hear them out (and maybe put a tarp down).
Sex can become increasingly complex, perhaps exponentially so, as one becomes more experienced. Each lover one significantly encounters can bring issues that result in a slight twisting of one's sexuality, so one may turn to pain, bondage, dominance, or any number of other kinks that can - but do not necessarily - make one a bit more distant from sex. When one feels one can't express the fullness of one's sexuality with a partner - because one loves them too much to smack them at the point of orgasm, because one feels shamed by one's carnal wants, because it would result in criminal charges in forty-eight states - one may turn to porn. This isn't always the case, but I have been a teenage boy and I understand that porn's seduction has little to do with sex I would ever want to have.
In addition to unconscious reactions to prior experiences, I would imagine much of it is built on primal instinct. For instance, cuckoldry fetishes could be based on reproductive dealings during the dawn of our species. In short, men are programmed to want to displace the seed of competition with their own seed and they will, as it were, jump more fervently to action if they believe their mate has recently been inseminated by an opponent. Interracial fetishes are indicative of this as well, plus a heaping dose of implicit racism.
Though the origins of these may seem atavistic and even outwardly offensive, it is important to understand that these fetishes do not make their possessors bad people by any stretch. None of this is intended as kink-shaming. Fetishes are not usually a matter of choice - you cannot be forced to have a fetish, though sometimes medication and therapy can alleviate a personally disturbing one - and one can indulge these safely and sanely (which is not something that can be said for all fetishes), occasionally by using porn.
Thus, Amber and I have unearthed the Ambush Discontinuity (a portmanteau of its discoverers and also refers to how much this can sneak up), the chasm between what one's brains/genitalia thinks is hot in porn and what one actually likes in real life. The only real bridge between the chasm for most people is talking dirty, as you certainly do not actually want your loved one to be choked with the gym socks of the girls who tormented you in high school (fetish atypical and impersonal, but stated for means of example).
I do agree that quite a lot of porn is a bit gross by design - most kinks are not mine and thus do not appeal - but I would not endeavor to legislate that which makes me uncomfortable. Like so many boys who grew up in the eighties, my first exposure to pornography was from a series of explicit magazines my older brother and his friends had pilfered. I could not have been much older than eight or nine - possibly I was a bit younger - but I did understand that this was exciting. I liked the idea of naked women, even if I happened to be a bit confused as to some of the specifics.
Despite what some conservative groups would say, I don't think this early exposure warped me. I am not plagued by anhedonia because I thumbed through dirty magazines any more than I've lost my taste for sushi because I had fish sticks as a child. Porn seemed to be an awkwardly lit fantasy world. At no point did I assume the girls in my grade would ever be interested in these staged scenarios any more than I assumed they would suddenly start tossing glowing flames at werewolves (I may have actively wished for the latter, since wizards and slayers were bound to be better company than porn starlets). In fact, I may owe porn a debt of gratitude, as it led me to HBO's Real Sex series which, while often tawdry and once detailing a clown orgy, contained better sex education than my schools cared to give.
Once I turned fourteen and started interesting actual girls, these magazines were left to molder in the basement. I gave any surviving magazines to my college girlfriend's artist roommate, who used them to make a collage of vaginae and breasts that would impress a serial killer.
I consider myself almost blessed to have pushed through adolescence and several relationships without any lasting psychic trauma that needs to be reenacted in fresh beds until I find catharsis. I know that I went through an emotionally loose period as a teenager ("Hey, I just met you and this is crazy, but here's my number, ARE WE IN LOVE NOW?") that thankfully tapered off and refocused once it came to the chemistry of intimate bodily fluids. If anything, the Ambush Discontinuity helped stave off the urge to do anything more scandalous than a bit of petting.
Soon in Xenology: The Ship of Theseus.