As the author of that Medium article I mentioned, there’s certain sets of terms that you can use to describe your relationship(s). Those terms tend to carry some ideas about how people behave. (Even in monogamous relationships this exists: “going steady”, “we’re serious”, “casually dating”, “married”.)
While the specifics may vary from relationship to relationship (no two marriages are the same, for example), I think it’s really important to remember that the terms describe your relationship. They aren’t a straightjacket you force people and relationships into, and definitely are not something you can use against another.
For example, let’s say you’re “seriously dating” (whatever that might mean for you) and your partner(s) act differently than what you expected. First, you make sure that everyone is on the same page about “seriously dating” and what that means. (Hopefully you did that already, but even if you did, do it again.) If so, then you do not get to say “BUT WE ARE SERIOUSLY DATING AND YOU DIDN’T DO THAT”. Instead, you get to change the description of the relationship to match the behavior of the people in the relationship.
That change isn’t always positive, but knowing that change happened and being able to honestly know where the relationship truly is makes it better for everyone.