I have heard of tampons being used analy in the Amazon, and such places, to keep assorted beasties from invading said orifice.
Of course, this would not apply to gay men, unless they are kicking it in the Amazon.
The link Washte posted suggests that this has nothing to do with the traditional use of tampons. It says “Many men…who are “fem” like to use tampons,” which suggests they’re doing it because women do, not to absorb fluids. Kind of like wearing women’s underwear, carried to an extreme. I suspect that this is either an urban legend or a joke on the part of the assistant, since I can’t imagine that the practice is so widespread that most of the people buying tampons were men. (Even if the store was in a large gay area, I’d think there’d be enough lesbians to make up for the tampon-buying men.)
I’ve always heard that men used tampons the same way women do - to soak up blood. I’ve heard that anal sex causes minor rips/tears and bleeding. I seem to remember hearing that this ‘fact’ increases the AIDS transmission risk for anal vs vaginal sex (though I remember hearing about this back in the early AIDS days - so it could be bs).