I'll chime in with the rest of the choire here, and state my opinion that men in thongs or g-strings are not sexy, not even if they have model-perfect bodies, asses to crack nuts on while a quarter is stuck in the crack, and a set of abs in front that I can wash my t-shirts on.
The thing about a thong is in my eyes, that it is too showy for my taste, too much craving for attention. I like men with more plain tastes in underwear, like boxer briefs, or squarecuts for swimming. I don't think those are boring picks, there is a pretty rich choice of both to pick from.
If you want further information on what us ladies like, regarding men's underwear,
feel free to read this poll/thread.