In the UK, and in many countries across the world, there are many different forms of marriage. These are all accepted as marriage, even though they aren’t “Christian”. On its own, that is enough to suggest that conservative Christians should not oppose the emergence of same-sex marriage. True, same-sex marriage isn’t part of traditional Christian beliefs, but neither are Hindu marriages, Jewish marriages, or civil marriages. The religious freedom that allows conservative Christians to celebrate their form of marriage means that conservative Christians should allow other people to follow their religious or philosophical views to celebrate marriage in their own ways – including Christians and other people of faith who want to celebrate same-sex marriage.
There is another reason for Christians accepting same-sex marriage, even Christians who believe that all same-sex relationships are wrong. A recent study showed that where same-sex marriage is allowed, the health of gay men improves. That means that by opposing same-sex marriage, you are effectively encouraging poorer health in gay men, which is hardly a Christian position to take.
Supporting same-sex marriage is not the same as approving of same-sex relationships, any more than supporting civil marriage is the same as supporting atheism. Opposing same-sex marriage is denying other people the religious and philosophical freedom that we all enjoy. Opposing same-sex marriage is wishing poorer health on a subset of the population. A Christian should support same-sex marriage, even if he or she believes that same-sex relationships are wrong.