It's because it's a major storyline character. The only way to enter a relationship with the roommate is with the roommate's partner quest series.
It will be a D/s relationship. Currently, all D/s relationships are treated as romance and will make the person more friendly toward you. I could be wrong, but I think this is a stopgap measure because the framework for D/s relationships isn't complete yet, so it was made a subtype of romance.