It can be sort of sexy... as long as it's done right.
If a guy decides to have long hair, he should at least take the time to maintain it.
If he doesn't wash it, and brush it periodically, he'll begin to look like a dirty old hippie... and of coarse, that's not very attractive.
I'm a guy and while I don't know about the attractive thing. I like my hair to be rather long because I like to tuck it behind my ears when I'm in deep thought and not being able to kinda throws me off.
Depends how long it is and how well kept it is, really. I don't really like it when it's a lot past the shoulders, but anything shorter is hot... Especially if thy have a nice face!
TBH I hate it. I do not like it what so ever. Although if the man I married had it I would not care because I love personality and what they are like not appearance.