Yours sounds like a "red herring" fallacy: the discussion was about monogamy being natural or not - not about the consequences or personal choices.

This fallacy usually happens when one dislikes a thesis, but isn't able to refute it (because they don't have any logical argument to oppose). So they divert the discussion towards something else.

Anyway:

1. Contraceptives. Doh!

2. Even in monogamy, couples of yore had many babies - even when they couldn't afford them. Nowadays we are more promiscuous, yet fertility rates are falling fast.

Hence your point is not only irrelevant, but also untrue.

It would have been better being honest and admitting: "I dislike promiscuity because..." - since I imagine that was your true motivation.