Gay isn't an activity. It's an orientation.
In context, it is a tag. And that can very well mean an activity in the game, that can be called gay, even though the characters might be bisexual.
It is just tags. One has to balance people looking for/avoiding certain content. If you have m-m action in a game, "gay" does the trick. Nuances that one or both of the characters are not oriented exlusively gay, are better explained in the description.
So, yeah, it is not what "gay" means, but it is one of the things the tag "gay" can mean.