At the very least, the couple WILL get a visit and be interviewed by the authorities. You may decline to press charges, but they will get a wake-up call that they can NEVER do that again.

Just to be a bit cynical... NOTHING against you, but to put it in perspective: They could have paid a prostitute the same amount of money to do the sexual asphixyating/watching to her, but they wouldn't have gotten their china dusted now, would they?

And also remember that in a fetish situation like that, there is a distinct chance that it will progress/escalate; what happens when just a little choking of a stranger simply doesn't turn them on any more... how long before their little game gets more serious?