There seem to be many mistresses and 1 or more reviewers of mistresses who have told of cases in which the mistress or mistresses in a double-domme session, after caning or whipping, will pee (or have peed) in a guy's mouth and expected or told him to drink the pee. I can think of at least 4 dommes who post (or, who have posted in the past, even if not regularly) at maxfisch who have told of such conduct or had reviews including such conduct. And it seems that few or none of the guys turn them down in the question of peeing. I don't think that there is a review or a report of guy refusing or trying to refuse peeing, but maybe it happens . . .
I can tell you that it has happened for me. I honestly don't remember the first time that I allowed someone to piss on me in a session because it has been so long and it wasn't a big deal. However I am sure that it occurred after I have been put in deep Subspace when I pretty much allow anything.
My limit for as long as I can remember though, has been body only. Some Dommes have tested that and wanted me to drink but it is something that I have refused multiple times from multiple different Dommes.
One Domme in particular, who I have a long-term and more personal (rather than simply Pro/client) relationship with, desperately wanted me to drink and worked diligently at it over many, many sessions for at least a year. She even went so far as to try to force me by having me tied up on the ground with a special balloon type gag that included a tube. This forced my mouth open but allowed her to piss into a funnel that emptied into my mouth.
Even in this predicament, I still refused to drink and literally forced it out of my mouth around the gag until I was choking and she deflated the gag.. Somewhat defeated (this wasn't a scenario where I asked for this and chickened out.. This was literally a forced scenario that she really wanted). She finally was able to get me to do it near the beginning of our next session when after really putting me in subby mode, she brought a glass to my lips and in a very commanding tone just ordered me to drink.
It was totally mind-blowing to be controlled like that and to this day she is still the only one I have drank for. But I would say that it took a half-dozen tries using all kinds of different tricks before I finally did it for her. Therefore, I can tell you that refusals happen and the Domme may be disappointed but never angry in my experience.