I've read several etiquette books, and none of them have ever mentioned the "don't separate a man's first name from his last" idea.
They have ALL mentioned "ladies first" as a rationale for things like "Gina and Bob Smith."
Interestingly, the book Honor & Respect: The Official Guide to Names, Titles, and Forms of Address by Robert Hickey, deputy director of the Protocol School of Washington, says to use "Bob and Gina Smith."
I think that teenagers are probably attuned to the "don't call me Mrs. John Smith" attitude (teenagers, in my experience, have a very heightened sensitivity to "fairness") and feel that they don't know all the rules. So I can totally see them picking the simplest thing.