I know that the conjugal act must be procreative and unitive in order for it to be in conformity with natural law and therefore not sinful. Why must it be this way though? Why does the Church put such a big emphasis on sex being within the framework of natural law? When elderly people where glasses, are they not going against natural law? Their bodies are designed to break down as they age and, along with many other functions, eyesight will worsen as well. So are they committing a sin by going against natural law?
I’m trying to play Devil’s advocate in order to get a better understanding on why the church believes sex must be unitive and procreative.