Earth's Moon is politically ruled from Earth, but has its own separate culture. Unlike the more permissive culture of earth, monogamy is the social norm. The low gravity leads to low fertility, with a couple usually experiencing many failed pregnancies before bearing a child to term. Due to their attentuation to a low gravity of 1/6 gee, few Lunies make a habit of travelling to other worlds.
Lexi Temple grew up on the moon and chafed at its restrictions enough to overcome her distaste for gravity and run away from home. Her parents blocked access to her trust fund in order to try and control her, but have no legal means, other than guilt, to try and recover their only daughter.