This protagonist is already committed to a romantic relationship as the story starts, or does so at the very early stages of the story.

So in the process of this story, if you aim for an another romantic interest this will cause all kinds of problems and guilt trips.

This also means that the protagonist won't be clueless about relationships like a kid encountering his/her first romance.

Having an Osananajimi that's head over heels in love with the protag, waking him up, cooking for him and hanging out with him at all times do not count. Since for the industry standard Donkan protagonists, she doesn't really exist.