Monogamy is the default though. They don't need to have her spell it out, when it's already shown naturally through her behavior and dialogue with the player.
That's a bugbear of mine. A relationship is monogamous / exclusive when two people agree it is, not before.
What? If you want to be poly more power to you but let's not pretend like monogamy isn't the default for the vast majority of people on this planet.