Straight is and always has been the default for society. The reason LGBTQ+ people have our own tags is that representation is limited so we have to search for it explicitly. Also, most liberals are actually rightwing and not very supportive of gay rights please get your terms straight before whining online about how you're forced to see gay people
This is about tagging. Since exclusion of tags does not seem to come along anytime soon, a positive tag might be a solution for some cases.
Or do tell, if you have game that is tagged "gay", do you expect to see much straight content in it? Tags should tell the user the focus of the content.
But yeah, since in the adult erotica context "straight" is pretty much the base, tagging it straight does not give much information, so nobody would bother tagging it that way, as "adult" alone pretty much does it . At least not the developer. User given tags might go the extra mile to tag like this.
Also, this would not be the solution for horror filtering. Being non-horror is the trait of ... well, every non-horror game. So tagging non-horror would not happen. But you could tag non-lgbt as straight.