Especially attractive to the same sex, but not the opposite.
The opposite sex will find them average at best, while the same sex may well come out of the closet as a testament to their one-sided attractiveness.
Suzie: hey Liz, what do you think of Jeff?
Liz: meh, he's ok I guess.
Football Star & Known Ladies Man: *glances over to see who they're talking about* oh my god. *forcibly makes out with Jeff*
Suzie: wow, i guess he's just Gay Pretty.