Single people can adopt, but not unmarried couples
But Conservative MP Robert Walter has laid down an amendment to these changes - to be voted on Monday - which would specifically exclude gay people from the arrangement.
Under current law, only married couples and single people, including gay people, are allowed to adopt.
Impassioned debate
Earlier on Thursday, a free vote on an amendment to the Adoption and Children Bill was carried by a majority of 155.
In impassioned debate on the change, Labour's David Hinchliffe, chairman of the Commons Health Select Committee, denied it was an attempt to undermine marriage.
"I have a deeply held personal belief in marriage," he said, stressing his one single concern was the interests of vulnerable children.
"Parents should be married'
Under present law, he said, "if the best interests of a particular child would be served by being adopted by an unmarried couple, this would not be allowed".
But former Conservative minister Ann Widdecombe argued that all parents should be legally married.
She said the legal responsibilities that come with marriage guaranteed children's "stability" and "security".
"If two people really want to adopt, if they are looking at the interests and security of that child, unless there is an impediment, they should want to marry," she told MPs.
Not 'anti-gay'
Tory spokesman Tim Loughton attacked MPs for using the bill to promote a politically correct "gay rights agenda".
The government should be putting the interests of the child first, not the wishes of the adult, he warned during a passionate speech.
"Too many people interested in this part of the Bill are in danger of putting the interest of adults ahead of the children," he said.
The Tories were not "anti-gay or pro marriage" but believed marriage provided the best foundations for adoption, he added.
'Political correctness'
Health Minister Jacqui Smith said the measures were about increasing the potential pool of adopters - not the right to adopt.
The Bill was about providing a stable family home for children, which did not necessarily mean with a married couple.
She added: "In the end this is not a debate about a right to adopt, it is not about political correctness, it is not about gay rights, it is not even about parents - it is about a child's chance of a family."
Single people
It is suggested that the courts allow an unmarried couple to adopt when they are 21-years-old or older.
The courts must be satisfied of the long-term nature of the couple's commitment to one another.
There must be provision to ensure the financial and practical security of the children in case the couple split up.
At present, a single person can adopt but an unmarried couple cannot adopt as partners.
Tory opposition Liberal Democrat health spokesman Evan Harris argued that unmarried couples, including gays, should be allowed to adopt, especially as individual gays could do so under existing laws.
Scientific research showed children fared much better when adopted by unmarried, including same sex couples, than in care, he added.
It is estimated that some 5,000 children nationwide are waiting for a home with an adoptive family.
The bill also sets tough penalties for those who avoid new safeguards when adopting from overseas, following the notorious Kilshaw case, where a British couple got around rules by going abroad.