If we do that, then people will be complaining it's NOT there. It's more of an informational disclaimer. I guess you can think of it as a "Trigger Warning". (I hate that term, but I can't think of a better one)...
You're definitely free to have that opinion, however, you having that opinion doesn't mean it's going to change the fact that the GL tag simply means that a relationship between girls (sometimes sex, sometimes not) exists as a major theme or plot point in said comic.
This comic DEFINITELY has yuri sex ー frequently at that ー therefore it gets the tag. We were nice enough to add a disclaimer saying it also has het sex, too. No matter the decision, Not everybody will be happy. Sorry you're not, but that's just the way it is...