The Church of England has backed proposals to give thousands of co-habiting couples similar legal rights to married people.
The Church believes marriage is still central to society's stability
Responding to recommendations from the Law Commission, the Church said it was "sympathetic" to law reform for couples where children were involved.
In other cases, the Church said more limited reform may be necessary "to prevent manifest injustice".
But it stressed marriage was central to the stability of society.
It also told the Law Commission it believed marriage provided the best context for bringing up children.
The commission has put forward recommendations which would allow unmarried couples the right to share in each other's wealth in the event of a break-up.
The Church responded that there was a "strong Biblical precedent" for protecting the vulnerable as well as for upholding standards.
The Bishop of Southwark, the Right Reverend Tom Butler, heads the Church's mission and public affairs council.
He said: "We're saying we will look, we will look at legislation that the government puts forward to help vulnerable children, who are caught up in relationships which break down, but the test will be that this does not downgrade the position of marriage."
Bishop Butler said it was "perfectly justified" in terms of public policy that marriage should confer particular benefits and privileges, "so long as adequate steps are taken to prevent manifest injustice".
Bu the Church Society said the Church of England's approach seemed contradictory.
The Reverend David Phillips, its general secretary, said: "It seems to me that they are arguing on one side for marriage and then undermining it with other proposals.
"Whatever the reasons, and however good you think might be the reasons for changing the law, it will undermine marriage and that will ultimately bring more injustice and problems, so I do not think this is the right tactic."
Two years ago, the General Synod of the Church of England said people in relationships "not based on marriage" might face issues of vulnerability and hardship which should be addressed by new legal rights.
But in June, the Archbishop of Canterbury said marriage had suffered a "long process of erosion" and the Law Commission's recommendations would add to a "prevailing social muddle".
Dr Rowan Williams added: "As soon as you define anything, you are creating a kind of status that is potentially a competition with marriage or a reinvention of marriage."