ass sex, like most forms of intimacy, happens on a curve. Most 10 year olds gross out at kissing someone of the opposite sex, much less having genital interourse...that goes away over time and, due to social conditioning, usually well before the opportunity to experience it comes along.
Anal sex (and all the oddities that accompany it from pain to poo) is the same way. With some notable exceptions, most guys I know who practice anal sex (passive or active) did not start out to be big fans of it. I know I sure didn't. But it grows on you to some extent or another, depending on your own unique sexuality.
Sexually explicit images and words can help this process along as well. This is why many men would crave fucking a woman's ass ("I'll let you put it anywhere you want" from Cruel Intentions) when to query them on their independent reaction to introducing their cock to someone's shitpipe might produce a gross reaction. Porn these days condition men to see the "hotness" in a girl who takes in both holes, and a certain accomplishment to being the guy to do it.
Similarly, many gay/bisexual men such as myself began to open up more to the idea of butt sex by watching incredibly hot images of men with rock hard dicks shooting loads of cum without touching themselves while in obvious throws of pain-laced ecstacy from being pounded by some porn-stud's huge cock. You watch enough of that, pretty soon you start to stiffen up to it, then you start to fastforward through all the groping and blowjobs to get to it, then soon enough you're fantasizing about doing it or having it done to you. After that, its just a matter of time before you exhaust sticking every household item you find to be reasonably safe up your ass and you need to move on to the real thing.
All that to say, ass sex is great. But its kind of like being abducted by an UFO, everyone who hasn't had the experience thinks your a bit nuts when you try to describe it.