This question doesn't seem to be posed for women, as there's not a lot I could do with an unconcious man, unless he just happened to have an erection. I might try really, really hard to wake him up, though.
Truthfully, though, I think it would depend on how intimately knowledgeable you are about each other. I mean, if you were so unfamiliar with this person that the discovery of their unconscious body with a note leaves you puzzled, then yeah, it's probably a good idea to let sleeping dogs lie...sure, pun intended. But if it's someone you've been with for many years and you know that this is something that turns them on and/or turns you on then I really don't see a problem with going through with it.
But, on a personal note, I would be alarmed just finding my SO passed out from drug or alcohol use. I wouldn't need a note to send up red flags.