Welsh Secretary David Jones has provoked criticism after saying that same-sex partners could not provide a "warm and safe environment" to raise children.
The Conservative MP made the remarks when asked on ITV Wales' Face To Face programme about the Government's plans to introduce gay marriage, which he voted against.
He said: "I regard marriage as an institution that has developed over many centuries, essentially for the provision of a warm and safe environment for the upbringing of children, which is clearly something that two same-sex partners can't do."
The Prime Minister's spokesperson responded by saying David Cameron rejects Mr Jones' claims about children in same-sex families and that he is in favour of same-sex adoption.
"The Prime Minister believes gay families can provide warm and safe environment for raising children," the spokesperson said.
The shadow Welsh secretary, Owen Smith, claimed the comments proved that "nasty party is alive and well under David Cameron".
He said: "David Jones' comments are profoundly offensive and he should apologise immediately."
Equality campaigning group Stonewall have also attacked the Welsh Secretary's comments as "offensive and inaccurate".
David Jones' voted against gay marriage, in a free vote earlier this month along with one hundred and thirty six Conservative MPs.
He defended his position in a statement: "I made the point of stressing that I was fully supportive of committed same-sex relationships. I also strongly approve of civil partnerships.
"I simply sought to point out that, since same sex partners could not biologically procreate children, the institution of marriage was one that, in my opinion, should be reserved to opposite sex partners."
The equal marriage bill is passing through Parliament after gaining a 225-majority on February 5, though it is likely to face more opposition in the House of Lords later this year.