Simple, shallow answer, which is much of the reason for me reading slash *sex* scenes: to quote Grace Adler, "Gay sex is hot." Period. It turns me on. Why? Hell if I know. Why do redheads turn me on? Why does light bdsm turn me on? Because it does.
More complicated answer, on the very specific issue of why I'd want to read or write something that "leaves me out of the loop" :
An example: I *hate* Buffy/Spike fic. Dislike it immensely on the show, but ain't bitching about it because the acting is superb, and the nekkidness is...well, there. Why do I hate B/S fic? Not because I hate Buffy. I don't. But when I read a fanfic, I *become* the viewpoint character (which is why pseudo-omniscient narrators bug the hell out if me). That means either I'm Buffy, in love with or having sex with Spike, or I'm Spike, vice versa.
Say I'm Spike, then. I have that mindset. I can do that. I can get into being the guy. Why doesn't the hetfic work for me? Because I'm not attracted to Buffy. I'm vaguely attracted to Willow, and would sell my computer for the chance to lick Amber Bensen's ankles, but I don't want to read about Spike (or Xander) being in love with them, either. Male-POV descriptions of sex with a woman don't turn me on., because even when written by women, they tend to seem idealistic, and *very* Mary-Sue. Describing BUffy/Willow/Tara as everything the author wants to be, instead of as Buffy, Willow or Tara. Moreover, they make me jealous, because why ain't it me?
Say I'm Buffy. And pretty much stop there. That's the biggest reason I don't like Buffyverse hetfic. I can't be Buffy. She's too pretty, she's too strong, she's too powerful and she's the center of attention. Seeing the two most beautiful blonde creatures on the show go off to make beautiful blonde love to each other does jack-squat for this short, fat girl from Indiana. It "leaves me out of the loop," as someone whom Spike or Xander would never go for, more than seeing either with a man does. I can't be Willow. She's brilliant and beautiful and powerful. I can be Tara, a little, only because she's *so* disbelieving about how gorgeous she is. But Tara is, was, and always will be queer as a three dollar bill, so I can't buy her with any male on the show.
I'd rather read a story about Spike alone in bed, jerking off, than read him having sex with a woman. The woman being there makes me have to think about *her* thoughts, feelings, etc, which are *not* going to match mine. They never do. And if I'm not seeing that woman as "Buffy" -- if I can disassociate from the identity of the character enough to put myself in her place, when I know I *can't* put myself in the real Buffy's palce -- then it's bad writing. Bad characterization. Bad story, Amy's putting you down and looking for another. If Buffy's going to be *in* a story I'm reading, I want her to be Buffy.
In general, I can "feel" Spike's or Xander's POV, much more than I can any of the girls'. And I want to read and write about men, because, well, they turn me on. Women turn me on too, but not as visually. Not as directly. So if the best POV for me is a man's, out of the available characters, and the best thing to read/write about are men's pretty naked parts, then slash is pretty much the ticket.
And, as others have said, the shock. The awkwardness. The coming to terms with this *wrong* thing, and realizing you like it. The embarrassment. The humor. The lure of the forbidden. Biggest: that two men, especially men who aren't primarily gay, heretofore, or are just discovering it, have to let their emotional barriers down *so* much more. It's so much more intense, so much more satisfying when they do come together, and admit they love/want each other.
Plus, have I mentioned, gay sex is hot?