Key point #1: There's quite a large percentage of gay male couples who DON'T partake of anal intercourse; some just don't find it enjoyable for any multitude of reasons, amongst which is the certain lack of spontaneity.. many just don't see the need, there's so much more to do with each other that doesn't involve quite so much potential pain or risk. Some are actively AGAINST the practice at all. Quite a few men just find it too uncomfortable to continue with it once they've tried it.
Aaaaand yet... very few of these other avenues are found in slash. Always TEH BUTTSECKS. Yeah, it has it's place and it's cool to read a story that does it well, but it's almost NEVER written the way it often actually happens.
When's the last time you read a fic where the guy went and gave himself an enema ("prepping") before sex (and it wasn't a kinkfic dealing with that specifically)? Right. Yeah, this is how you avoid getting (much) you-know-what on your cock. And no, I have no need of seeing that particular aspect of it ending up in the fic either; I'd really rather not, but it's a fact of the activity. Santorum happens.
Some positions are less likely to cause this to happen as well, and that's almost never addressed either. Personally I truly dislike any scat mention in fic; Not My Kink (tm). I'm certainly not advocating for more of such; just noting that it's a reality that most dislike and thus go to quite some lengths to avoid.
I know I don't have to go into The Lube Rant yet again, you guys have heard that from me enough.
Even so, even with the slickest lube cash can buy, rough anal sex has SUCH a risk of causing damage one way or another. All it takes is one ill-advised encounter of letting someone pound the hell out of you, and you are out of action (derp) for what can be weeks or even months. There is NOTHING sexy or fun about an internal fissure. Go look it up. And yet there's fic after fic after fic of multiple rounds of banging away on each other.. yeahh, no.
Again, the lack of spontaneity matter comes up in rimming... Go wash your ass first, dammit! Yes, there's always someone's kink to consider, but for the most part? Eeeyyuugghhhhhhhhhhhhh.
Yes, fresh sweat can be nice. Yes, even in the crotch. But around back? Not so much.
A lot of guys take a bit of extra care to keep as clean as possible in general if this is an active part of their sex life (hello, baby wipes...), but still.. much like the Gentleman's Wash, there's going to be that "back in a moment" point in time.
Now.
<b>Fic isn't meant to be reality.</b> These aren't criticisms or denunciations of the many excellent stories out there which I've enjoyed reading. This is just answering the question inasmuch as my own experience allows. As always, I speak for myself, YMMV. I'm sure there are guys out there who can take a beer-can cock jackhammering with just spitlube at warp speed and live to tell without a single problem, but I ain't one of them and I don't personally know any either.