No, Gene Simmons did not take a piss on stage last Wednesday during the KISS concert in Denver. While he appears to be doing just that in a video posted online, what the video does not show was him washing off the fake blood he spits off his costume. Asked about it in a tweet, Simmons says, "This is hilarious. I gotta try this in a show. Wow."
Well Gene, if you really need to go, perhaps you may want to consult with Ozzy Osbourne. Legend has it that when he pours a bucket of water over his head in concert, he is only doing to to distract the audience from the fact that he is urinating.
Video in which it looks like Simmons is urinating on stage starts at 3:42.