As I said, the tags are about the kinks, and m/m will be liked by different people than f/f. The idea is not to distinguish in terms of worthiness or similar, but in terms of people trying to find games they may enjoy. Trying to use the same tag for them to avoid discrimination will be counterproductive.
As for the term, again as I already said, it is technically correct that "gay" can refer to both male and female same sex stuff. However, it is used very often, even within the community, to refer to male partnerships, since there is a different word for females. Again - LGB is Lesbian, Gay, Bi (yes, there are more groups, but not relevant to this discussion). If you have a different suggestion as a term for "gay", please come forward, renaming it shouldn't be the problem.
It doesn't.
Here's the list and definitions. The basic idea is that the tags are not about the orientation of the characters but about the sexual actions depicted.