It's a strange thing - that people really believe they can redefine marriage, a sacrament founded by God.
No matter what society decides, no matter what laws may be rewritten, no matter how people behave as if it were true, no matter how much they pretend, no matter what people believe, no matter how much coercion they try to bring on others to believe something different, no matter how much they succeed at that coercion, they can never make a marriage between two people of the same sex into a spiritual union in the sight of God.
I believe same-sex marriage is a sentimental act - a fiction - play-acting.
It is in a way a sweet sentimental act - it expresses the wish of gay people that their relationship could be more than it is - but the Bible teaches that it is not and never will be the real thing, real marriage, in God's sight.
Believing that same sex marriage is not real in God's sight should in no way stop any Christian from being kind and polite and friendly to gay couples - in fact, believing that same-sex marriage is not real in God's sight will not stop any Christian from loving same-sex couples with Christ's love, any more than disagreeing with anyone about anything should stop us from loving them now.
All true Christians will see this the same way, for our desire is to show God's love to everyone, regardless of what they think, believe, do, regardless of the state of their hearts, or their actions. (False Christians, who compromise with the world, will of course hate the true Christians for insisting that gay marriage is not marriage in God's sight. They won't be able to help themselves.)
But Jesus defines marriage as between a man and a woman. In God's sight, this is the only sort of marriage that is recognised as a marriage. No one can change this and no one ever will.
Furthermore, God provides instructions for the case where a marriage is not a marriage - Jesus tells how to view this.
It has also been said, ‘Whoever divorces his wife must give her a certificate of divorce.’ But I tell you that anyone who divorces his wife, except in the case of immorality, forces her to commit adultery. And whoever marries a divorced woman commits adultery. Matthew 5:32
It has been said that this teaching of Jesus' was a slight dig at Herod's marriage to his ex-sister-in-law, which was clearly immoral in the sight of God, even more so since Herod was divorced. Whether or not this is the case, it is permissible to get divorced in the case of 'immorality', i.e. a marriage that is immoral in God's sight.
It is interesting to reflect on this fact. If someone is in a same-sex marriage, Jesus does not hold them to their vows.
(A few minor improvements, 9 July 2017.)