Oh yeah it's a buzz kill alright, total limpener. But even very experienced and extremely well prepared power bottoms can have it happen from time to time.
The reasons why it's happened to guys I've been with in the last three-or-so years:
1) They were in a rush: there's a sequence here that's got to be followed. Douche, expel, wash, wait, repeat. The sequence takes at least an hour, sometimes longer.
2) They were not thorough: they skipped a step, usually the last two.
3) They were trying to be spontaneous: anal sex is not a spontaneous act. It requires preparation.
Each partner has the responsibility to bring all he or she can to each encounter. I have pretty significant digestive issues of my own: both the meds and HIV itself wreak havoc on someone's gut. If I'm not certain that I can enjoy an extended period of time focused on pleasure rather than what's going on in my colon, I skip it. I expect no less from a bottom.
Of course accidents can happen, and when they do I'm gracious but firm. I won't go near his hole again that night, and will think hard and long before agreeing to a follow-up. It's a sea/fish thing.