I do not think C. S. Lewis ever directly wrote about gay unions. But he did write about the way in which the government might be connected with defining that which is marriage and that which is not.
In his unforgettable book, Mere Christianity, C. S. Lewis penned something which directly corresponds to the subject in our courts and churches now about defining marriage. Lewis was writing about marriage between divorced people, though the can equally be relevant to unions between two homosexual people.
Here is what he wrote:
Before leaving the question of divorce, I should like to distinguish two things which are very often confused. The Christian conception of marriage is one: the other is quite the different question-how far Christians, if they are voters or Members of Parliament, ought to try to force their views of marriage on the rest of the community by embodying them in the divorce laws. A great many people seem to think that if you are a Christian yourself you should try to make divorce difficult for every one. I do not think that. At least I know I should be very angry if the Mohammedans tried to prevent the rest of us from drinking wine.
My own view is that the Churches should frankly recognize that the majority of the British people are not Christian and, therefore, cannot be expected to live Christian lives. There ought to be two distinct kinds of marriage: one governed by the State with rules enforced on all citizens, the other governed by the church with rules enforced by her on her own members. The distinction ought to be quite sharp, so that a man knows which couples are married in a Christian sense and which are not.
It is time for Christians to understand that the United States Government (or any human government for that matter) is not the one to enforce biblical rules and laws. Nations produce their unique guidelines which they believe can help bring peace, safety, and stability inside their borders, however these laws are sometimes at odds with the directions and directions of the Bible.
Even though the state makes something “permissible,” this does not mean that it is now legal for Christians, for we are responsible to observe both the laws of God and also the rules of our own government, using the laws of our God having precedent in our individual actions over the laws of our nation. And in addition we really should be cautious about trying to get our administration to enforce Scriptural rules on everyone in our country, for if it can be carried out with “Christian” commandments, it could also be enforced at a later time with “Muslim” commandments, or “Mormon” guidelines, or whatever religion happens to be “power” at that time. C. S. Lewis provides the illustration of Muslims and their restrictions on consuming alcohol. None of us would desire this, but in a number of ways, this is precisely what Christians are wanting to do on our culture when we try to force our neighbors to embrace scriptural rules about the gay lifestyle.
In some places Muslims are attempting to implement their Sharia law upon all people in that nation, just as within the USA some Christians are attempting to implement our own principles on all the people in our country. If anything may be learned from history, we understand that it is dangerous to everybody when any religion takes up the sword and attempts to implement religious laws on everybody.