Well, this is an interesting question. For issues of this sort, one could do no better than to lean on Miss Manners for advice.

I believe I have previously commended her Miss Manners' Guide for the Turn-of-the-Millennium to your attention. Older but equally pertinent and timely tomes include Miss Manners' Guide to Excruciatingly Correct Behavior, Miss Manners' Guide to Rearing Perfect Children, and Common Courtesy.

As it turns out, this specific social situation is covered briefly but succinctly in the most recently published volume under the chapter titled A Sampler of Problems Mrs. Perfect Never Had to Face: "Guests must always be warned before attending any event at which people are going to take off their clothes." Case closed.

I must confess to wondering, though, which side of the argument you sit on.