I have suggestions for two tags, but before I get to them I'd like to bring up a few things that give it context.
One of the most requested features on Itch is a more robust and intuitive way to exclude games based on tags. I imagine that it must be harder to do with the way Itch is coded than it would seem for us users, because I really do imagine it would have been implemented already if it was an easy task.
But, in the meantime, there's no robust exclusion feature. And I'm not sure, but I don't think we can exclude multiple tags at the same time.
That's the first part of the context. The second part is... If you take a look around Itch you'll see that LGBT and adult games do really well here. I work on adult LGBT games myself so of course I think that's great. In no small part this is because Itch's tagging system works exceedingly well for LGBT content, to such a degree I'd say it's a textbook example of good platform design for LGBT creators.
We have tags such as Gay, Lesbian, Yaoi, Yuri, Bara, Amare, Trans, LGBT, LGBTQIA, Queer... It gets to a point where we can even pick specific flavors of LGBT content. And that's great to me.
Another thing we have here is marking a game as NSFW in the metadata (which is different from using a tag) and tags like Adult, Erotic, etc that clearly denote there's adult sexual content. At the same time, while adult LGBT games can pick, let's say, Queer + Lesbian + LGBT + Adult in their tags, if your game focuses on straight content you only really have the Adult and Erotic tag.
In practice, game devs making heterosexual content use the Adult tag as their main tag to indicate their game has straight content. At the same time, LGBT game devs use Adult to indicate their LGBT games also has adult content. Which means that, in practice, there's no single tag that indicates "this is the place exclusively with heterosexual content".
Anyway, so far this system works well enough. It would work better if there was a robust exclusion feature, of course, because it'd make it easier to find games down to an even more specific point (games that have X and Y, and do not have Z). It would also make it easier for non-LGBT users who just don't want to be shown gay visual novels, which I sympathize a lot with because I and many others would love not to be shown Horror games. In general, I think it's great and ethical for platforms to give users features to let them exclude content they'd rather avoid.
Now... Again, this system works. But when we put all of these things together, a few issues pop up.
The first of them is that there's no single tag that indicates straight content, clearly and broadly. Aside from Adult and Erotic (which are used by LGBT devs too), there's Hentai (which is straight but also denotes anime content) and Eroge (which is not limited to straight content.)
If Itch had a robust tag exclusion feature, none of this would be an issue. Users could just check for Adult games without the LGBT tags (LGBT, Lesbian, Gay, Trans, etc) and they would be happy, knowing that the results would overwhelmingly be exclusively straight content. But we don't have that feature, and we don't have a tag for Straight content, and put together those two become an issue.
There are some issues with adding a Straight tag, but I'll get back to them later. Let's move to the second point I want to bring up, and this is the one I find the most important.
The second issue is that if you want games with bisexual content — either story-focused games where the protagonist is bisexual, adult games with partners of all possible genders, etc — you have nothing to really find them. If we had a Straight tag we'd be able to mix them all up — search for Gay, Lesbian and Straight at the same time to get the games with bisexual content — but that wouldn't work now because no one uses the "straight" tag.
Now, beyond that point of mixing tags pragmatically to find content, I think we should look at this with a more careful lens. I think we should really consider that, much like we are seeing Gay and Lesbian games doing great on Itch as their own little communities, the same could happen with games intently focused on Bisexuality as a topic. The "bisexual" tag has 31 games right now, which isn't bad at all! I think there are more games with bi content that just weren't tagged that way because there's no official tag for it.
As a game dev making adult content, our consensus when it comes to games with bisexual content is that there are significant differences with how they should be done -- they are different from gay games in how we introduce and manage the characters, what content should be mandatory or optional... Things are significantly different and require a different craftsmanship.
It's also worth bringing up that it's Pride Month now, which I think is a very auspicious moment to have this discussion. To an extent my suggestion is a pragmatic one -- how we can improve things given the lack of a robust exclusion feature. But there's something to be said about Itch having tags for Bisexual and Straight content too. I think it would make it a more welcoming place, and maybe make Itch more usable for the (many) people who (quite fairly) don't want to be shown LGBT visual novels all the time.
Now, I did mention I'd get back to a possible issue with the Straight tag. It's very simple: only 13 games use it, less than the "bisexual" tag, despite there being many more straight games. I know that the number of games using a tag is often considered for this argument. But even if there are few games with the "straight" tag I'd really recommend you guys consider it too. I think it might have a good snowball effect over time. And it would convey a very welcoming message, I think, that there's no reason for users to be mad because someone is making games they aren't into and, unwittingly, making it harder for them to find the games they want.
So, in summary: making a Bisexual and Straight tags would make things better, particularly in the absence of a robust exclusion system.