At this stage I have read quite a few books and innumerable articles about the same-sex marriage debate. One of the 
best of the books is a new one called simply, Debating Same-Sex Marriage.
It is co-authored by John Corvino, a lecturer in philosophy at Wayne State University in America, and Maggie Gallagher of the National Organisation for Marriage.
Corvino argues in favour of same-sex marriage and Gallagher against and both authors put their case well.
However, reading through the opposing arguments in this and other books, the unavoidable and irreducible conclusion is this; the two sides cannot agree on what the core purpose of marriage is.
One side says the most important purpose (not the only purpose) is to encourage mothers and fathers to raise their children together. The other side, the pro-gay marriage side, says this is not the most important purpose of marriage.
Both sides agree that a very important purpose of marriage is to encourage companionship and commitment.
With regard to children, however, pro-gay marriage advocates say what is important is supporting and encouraging committed, ‘quality parenting’, and not motherhood and fatherhood as such.
This being so, they do not believe we need an institution that is aimed mainly at encouraging men and women to raise their children together.
So this is the impasse the debate about same-sex marriage reaches. Either we believe we need an institution that gives special and unique protection to motherhood and fatherhood, and recognises that there are important differences between men and women and mothers and fathers, or we do not.
This is what the public itself must ultimately decide, but a society has to take a very strange turn to convince itself that it no longer needs an institution that is expressly designed to bond mothers and fathers to their children.