"He was night putting. Just putting at night......with the 15-year-old daughter of the dean. And her lesbian friend."
Because that is exactly what many people think of when they hear it or read it. That's how language works. You may not like the meaning people take from a given word, but that doesn't change the fact that they will take it that way.
Dan Savage, who purchased the original Dear Abby's desk at auction and writes his Savage Love columns on it for inspiration, approves.
Threesome has been co-opted, as gay has been for a 70 or so years. Abby might have used "trio" as a better choice
Serious reply time: I think the context makes it fairly plain that the threesome wouldn't be sexual. Unless she intended to advise them to have a menage a trois on the dance floor. Or the prom band is really a collection of porno soundtracks.
That's terrible advice by Abby. She should have said that they needed to invite a fourth to videotape the evening.