I think that's actually a very good criticism and would be a good topic for a rules update discussion on any of the Hentai communities that don't include it already. A simple and consistent tag system would be a good addition to the rules and prevent this type of situation.
Edit since I can't copy paste sidebar text in Jerboa apparently:
Below is the tag system used by most Yiffit communities and could be used with success here as well. There are some further ones for the more specific communities, but I don't think we would need anything more in depth than this.
"Please indicate the gender pairing with one of the following tags in the title:
[M] for a single male.
[F] for a single female.
[M/F] or [F/M] for pictures only containing straight interactions.
[M/M] for images only containing male on male interactions.
[F/F] for images only containing female on female interactions.
[Bi] for pictures in which the characters are involved in multiple types of straight, gay and/or lesbian interactions."