Dr Peter Saunders commended the Archbishop of York, Dr John Sentamu, for taking a strong lead in advocating marriage.
Dr Sentamu told BBC Radio 4's Today programme this week that while he would "not diminish" same-sex households, marriage remains the "bedrock of society".
Marriage has hit the headlines in recent months as Westminster and Scotland seek opinion on plans to change the legal definition to include same-sex couples.
The Scottish Government's consultation came to a close last month and a similar consultation is to be launched by Westminster in February.
Churches and Christian organisations in Scotland campaigned fiercely against the proposals and it is expected that the Westminster consultation will prompt a similar effort.
David Cameron, Nick Clegg and Ed Miliband have all expressed their support for a change in the law.
Dr Saunders urged other Christians to speak out against the proposals.
"Some people may choose to live in civil partnerships, and under present law this is now legal (although not moral). But no one has the right to redefine marriage for the rest of us," he wrote in his blog.
"Let's do everything possible to make sure they don't. Marriage is special and unique. Marriage is marriage – one man, one woman, for life.
"It is not for governments to define – but simply to recognise."