It comes down to respect, and being a decent person to another. Just as I won't shove my foot up your ass without asking(or without someone deserving it), I don't expect to suddenly be choking on a guys dick, because he thought it would be sexy, but didn't want to 'kill the mood' by asking first.
It has happened before(not with a client, thankfully) and had he taken the time to ask me first, he would have known that trying it was not something I was ok with. I love sucking cock, but I can only take so much at a time. Shoving yourself down my throat without warning will just piss me off.
Ask first, it won't kill the mood, I promise! As Gen said, one of the main rules of BDSM is that both parties must be consenting and communicate their desires before acting on them. I like being tied up and flogged until my ass if red and I have bruises for a few days afterwards, but only if asked permission first.
In any good D/S situation the Sub is always the one with the true control. They need to give permission for the Dom to do things, and hold the right to stop the play if they grow uncomfortable.
