Thalassa

The setting takes place in a world where monster/non-human races coexist with humans. Centuries ago the males of these races slowly stopped being born, and to survive, the female ones of the races started to marry humans. If the child is male, they are mixed human and whatever race the mother is, if female, the child just takes after the mother. The time period is a mostly modern setting. You are a man that has been selected by the government to marry monster women. You can be human or mixed race. To keep their population up, marriages consist of multiple monster women usually.