People are more comfortable with freedom, even when it comes to their relationships. People see monogamy as restrictive and ultra-demanding, so they often reject it. This is not proof monogamy is not natural. No one can argue convincingly that polygamy is anything but an aberration which exists only because of oppression and power disparities. No one can look at serial monogamy or promiscuity without having to deal with the collateral damage of these choices. If monogamy is not natural, nothing is.