1 is a playable counterpart to a race, people only demanded it for personal reasons (omg I want a pretty cat boy!) So Yshnal is correct, if lore says otherwise, that's how the design is. Look at what they sacrificed to give us male cats and female roes, they didn't simply decide to get rid of belt models and scale character models down to almost PSX style builds.
That's a poor counter argument and you explained why yourself: "people only demanded it for personal reasons." It was not in SE's original intention for the game, so far as we are able to tell. The change was still made. This is a game not unfamiliar with Lore revisions. Where were Soul Crystals a couple years, for example? They weren't put in until players said they wanted the more traditional roster of jobs present. Bam. It was done.

Anyway, the entirety of your argument—and Yshnal's argument—is based around the existing world of Hydalaen as we know it which is made by real people. If those real people decide to include heterosexual marriage but exclude same-sex marriage it begs the question: why not one without the other? This game is not created in a vacuum. It already draws from real life. Hydalaen has a lot of things that the real world has: transportation, money, government, war. So you're telling me it's going to include heterosexual marriage but not same-sex marriage because, lol, those Eorzeans are just built differently? Or something? Makes no sense.