@Equilibrium, your points are valid but it still is presented as an indentured service which often wasn't much better than slavery at many times. At best the beastkin are second class citizens if even that. Judging by the incidents where the protag was asked who her master by strangers indicates that any existence outside of state run facilities without some human acting in that role is not allow in their culture. This means that we have a system, while not slavery and theoretically allows for some discretion by beastkin in choosing who they serve, is ripe for abuse. Choosing to leave the service of a human would likely relegate the beastkin to becoming incarcerated in some sort of state facility until a new master can be selected and they might see that as a worse situation.
While we haven't been shown if a beastkins mature further or if they are naturally neotenous (that is they retain child-like features into adulthood) it feels like the protag is still pretty young so I agree that their relationship development is a little weirding