Not having been a part of a lot of these conversations I can imagine a dialogue.
partner #1 : You know, I'd really love to spank you/tie you up/etc/etc/etc
partner #2 : (offended) You pervert/sicko/etc get out of my house...
partner #1 : What? (lying) I was kidding, calm down. Learn to take a joke.
You clearly subject yourself and your relationship to a lot of risk to bring this sort of thing up. With a stranger/etc it would seem there is less to lose. You are either accepted(great) or rejected(no biggie, not really a concern as you don't know them and don't intent to ever spend more time with them). With a spouse, long term GF/BF, things can get touchy and hurtful depending on how offended they are.
I guess its best to try this sort of thing very early on in a relationship so there is less concrete to lose?(perhaps more potential to lose)