Mr Justice Mostyn, a High Court family judge who was previously a well-know divorce lawyer, says there is no evidence to show married couples are more likely to stay together than cohabiting couples.
He told the Daily Mail: "It is not the role of the state, in my humble opinion, to go round telling people how they should form their relationships.
"I do not support two classes of adjudication depending on whether there happens to be a marriage.
"I support the extension of the existing system...to the unmarried, warts and all."
If a married couple decides to divorce, there is a legal obligation for the wealthier partner to support the other partner and their children.
Conversely, if a cohabiting couple separates, the wealthier partner is only legally obliged to provide for their children.
Marriage also provides certain tax breaks. For example, if a married partner's spouse dies he or she does not have pay inheritance tax. However, couples who are not married do.
Harry Benson, from the Marriage Foundation, told Premier: "The fact is that, if you want to stay together as a couple, the reality is that sooner or later you need to get married.
"Ninety-three percent of all those who have 15-year-old kids who still manage to stay together are married.
"They're [unmarried couples] a vanishingly small minority; only one in five actually make it all the way through. We hear an awful lot about divorce rates being high, but it's nowhere near as high as that; we're talking about 80%.
"I mean he's [Justice Mostyn] missing an enormous trick here. The state pays a fortune in picking up the pieces of family breakdown, the huge tragedy which affects 45% of teenagers today...
"Family breakdown is a massive problem."
"Our problem today is not divorce. It's the trend away from marriage and not committing to one another and then ending up splitting up and we end up as a society picking up the pieces, and it doesn't need to be like this.
"There's a good method out there which works pretty reliably well, and it's called marriage."