Ursinus Intro Ethics Blog - Section B

During the discussion of "That Matrimony Should Be the Union of One Man with One Woman," Aquinas seems to be saying that lines of reasoning can be natural instincts. I don't agree with this. Let's look at what he says: first, he says that all animals desire to "indulge at will in the pleasure of copulation" and anything but monogamy will result in deprivation. While the urge to fight for sex may be instinctual, the reasoning that monogamy is necessary to have copulation at will does not seem to be. Further, men naturally want to be assured of their offspring, and this is why they want monogamy. However, it doesn't seem to me that these things are instincts: only reasons. Perhaps "natural instinct" is a line of reasoning instilled into every member of a species by God? Otherwise, I can't tell how a specific line of reasoning is an instinct. But then how can we explain the existence of people who do not desire monogamy? What if God does not instill particular lines of reasoning, but "reason" itself, which guides everyone to the same conclusions? Has everyone with reasons to reject monogamy been given faulty reason by God?