It probably would require a lot of work to add it to the game for many reasons stated previously in this thread. I'll just quote myself:
I highly doubt this could be added in a post-game patch. I mean, it could be done, but it would require a ton of work.
First, they'd have to add a way to define a Mii's sexual preference. That's already a multitude of big design decisions. It could be a checkbox on the Mii creation menu or by observing your choices of interaction with other Miis.
Then they would have to create additional barriers to prevent incompatibile preferences from falling in love with each other (because you cannot directly control your Mii's feelings), which would also affect how romantic interrupts work. They'd have to create all new romantic scenes for each type of pairing (because it would be stupid and wouldn't do the subject justice if they simply just replaced one gender for another).
They would have to create adoption as an additional option to create children. Probably a ton of other stuff, too.
I guess they could just make every Mii bisexual, but I don't think that's treating the subject with the care it deserves.
But I'm also just guessing. I didn't work on the game, but from what I've played of the original Tomodachi Collection, seems like it'd require some significant changes to how the game works.
I'm wondering if they make a sequel, maybe they could give you direct control of your Mii, making a lot of the issues moot, since you'd be able to get your Mii to do exactly what you want and start relationships with exactly who you want. I'm guessing that's why it's not really an issue in games like "The Sims," because your male Sim isn't going to go make out with the guy next door unless you tell him to. But maybe giving direct control of your Mii takes away the charm and uniqueness of the game, turning it into sort of an Animal Crossing clone.