I like what you said about setting boundaries. Would you go so far as to explain why you're setting the boundaries and that you have shy bowel?
In my own experience, I've found it helped, though people don't always quite get it. Sometimes though, the opposite happens, and people go, yeah I get that too. Or 'one time I was away on a meditation retreat, school camp, etc etc'. At which point I say imagine that happened every time you went away/had someone stay at your house/ stayed over at someone else's etc.
Level of severity of this condition can be the difference between it being an irritating inconvenience... or utterly blighting.