There's a part of the degredation of this kind of thing that speaks o the deepest most deprived areas of my mind. The way I see it, if your a victim if rape, being marked by the attacker in that fashion simply acts a final humiliating touch. In fantasy and stories it's a definite feature from time to time. As a sub, in fantasy and real life I think it's a turn on and act of ownership for the dom. I would draw the line as pee and spit, not really into anything more than that. I love being spat on during a rough fem/dom session or just during a hard love making session as well.
In real life, as far as urination goes, It's happened once from another girl, though it was in a shower so it wasn't messy or anything like that.