My theory of people is that most aren't naturally mono, or naturally poly, but naturally hypocritical.

While many mono people genuinely feel no desire for other partners, many - perhaps most - suppress this desire because they think this is expected of them, and also because they couldn't handle their partner having such a desire.

While many poly people genuinely feel no jealousy about the romantic and sexual actions of their partner(s), many - perhaps most - strive to overcome this jealousy because they recognize their own desire to be with other people, and wish to extend the same freedom to their partner(s).