This is your resident Thomist bringing you a minute from the Bellowing Ox.
We all have heard of the natural law, but for most it sounds like an ethereal myth sometimes referred to by philosophers. It is actually grounded in every day experience. One of the precepts of the natural law that Thomas mentions is the education and rearing of children. This is something that all animals – including humans – do. Males and females pursue each other in all animal species for the sake of procreation.
Human beings experience this precept by sexual attraction. Men chasing women, and women chasing men evidence this quite explicitly. Unfortunately, our culture has perverted this precept or outright ignored it. Sex no longer includes preservation of the species, but is merely about self-satisfaction. We have divorced sexuality from one of its intended purposes, the family, and we are reaping the rewards.
There is indeed a purpose to marriage, both societal and individual. The Church has said that the purpose is unity and procreation. By rendering marital relations sterile, as “gay marriage,” sterilization, and contraception do, we contradict the 3rd precept of the natural law. We do not merely violate Catholic teaching, we violate the teaching of our very natures, which is why our society is ripping apart at the seams.