Mark Silk draws us to this passage in Mere Christianity that shows C. S. Lewis would oppose state prohibition of same sex marriage:
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 the quite 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 Mahommedans tried to prevent the rest of us from drinking wine. My own view is that the Churches should frankly recognise that the majority of the British people are not Christians 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.
Silk observes,
A great many American Christians seem to think that if you are a Christian yourself you should try to forbid same-sex marriage for every one. Lewis, were he alive, would not think that. … [Ironically,] [f]ew books have influenced conservative Christianity in America more over the past half century than Mere Christianity, the little volume by C.S. Lewis that consists of a series of lecgtures he gave on the BBC during World War II. Its enunciation of a generic Christian faith and practice has held special appeal for evangelicals: Five years ago Christianity Today ranked it as the third most important book in shaping evangelicalism since the war.
Read it all.