A Canberra couple have vowed to get a divorce, ending their "sacred" 10-year union, if Australia allows same-sex couples to legally marry.

Nick Jensen and his wife Sarah believe widening the definition of marriage to include same-sex couples threatens the sacred nature of the union and leaves the door open to polygamy.

The Christian couple have been happily married for over a decade, have no intention of separating and hope to have more children. For all intents and purposes they have a healthy marriage.

But in a novel protest against any successful move to legalise same sex-marriage in Australia, Mr Jensen wrote in an article published in Canberra CityNews on Wednesday that they are prepared to divorce.