Sure they could. It would just bring in droves of people bitching about a LI already being in a relationship and its a good way to kill off momentum a decent game has going for it.
That kind of stuff 100% needs to be explained in a dev post really early on in development so people know what they are getting into because its one of the tags/genres that is incredibly divisive.
Any game that has content like that added partway into development without clear communication from the get go always turns into a mess without fail, and derails every single update post as new people play the game.
And just to be clear, its entirely possible that isnt what is happening in this game, as we dont have confirmation why she is acting so fucking weird when the MC brings up her being in a relationship. Theres just limited things that make sense so its an educated guess.
There are also questionable planned tags with the dev confirming no lesbian content, but saying group sex is going to be a thing.