Yale historian John Richard Boswell wrote in his book, Same Sex Unions In Pre-Modern Europe, published in 1994, that priests used to marry couples of men.
An article on Care2.com explains besides that the Christian religion not only tolerated, but also celebrated unions between same-sex people: "In the famous St. Catherine’s monastery on Mount Sinai, there is an icon (the picture above) which shows two robed Christian saints getting married." In the 4th century, Serge and Bacchus, the two saints, were married and guess who was their best man? Jesus-Christ himself! No kidding!
The article tells that a little later, in the 14th century, men and women couples could marry in Serbia. There was an "Office of the Same Sex Union".
Finally, in France, there were contracts of "enbrotherment", a kind of contract of marriage in the Middle Ages for men who pledged to live together sharing ‘one bread, one wine, and one purse'.
Until the 18th century, gay marriage was actually accepted.
If you still hear people opposed to gay marriage saying they want to preserve the traditional marriage or by religious conviction, you know what to answer.