I understand your plight, but also want to offer from a promiscuous bottom's perspective: I am usually 100% clean when I take cock, even without elaborate preparations (just lucky that way). However, shit does indeed happen. It is very rare that I am dirty when I get fucked - in fact, I will be upfront before hooking up if there is the possibility. Yet, despite the best efforts, every now and then it may be dirty. I don't play it off casually like your bottoms seem to - I apologize profusely. Most tops and I acknowledge that the moment has passed, but we can still end friendly. There have been a few tops through the years, though, who have been real assholes about it, which is not cool.
I would recommend that you be very clear before hooking up, like others have suggested. And if it happens, be clear but POLITE that it's over for you. Don't play it off casually like it's no big deal, but don't be an asshole about it, either.