As far as I can tell, these are currently serving the same purpose, and have about equal use. Should they all be shifted to one or the other? Is there a preference which?

English is not my primary language, but I don't think those tags are necessarily for the same thing.

Isn't 'fly' used only for trousers?

In this case "open fly" may implies "unzipped", but not the other way around.

see http://danbooru.me/post/show/282102

Agreed with gsimon. Open_fly implies unzipped, but unzipped can refer to anything with a zipper, while open_fly refers only to the zipper on the front of the pants.

I'll go ahead and do an "unzipped -open_fly" search to add the latter to any relevant posts and suggest the implication:

open_fly → unzipped

Yeah. The comment was mostly from seeing a few posts where it seemed that unzipped was being used as a replacement for open fly. Guess I just assumed it was a more rampant trend than it actually is.

Added open_fly -> unzipped implication. And yes, what gsimon and roast said about the two tags is correct.

I'm thinking open_fly should not imply unzipped as some images may simply be unbuttoned, with the zipper still zipped, as in post #1070465.

I'm not sure that would really count as open_fly, though.