David Cameron denied being "at war" with single parents today as he faced questions over planned tax breaks for married couples.
The Tory leader was accused of "discrimination" as he delivered a speech to members of lone parent charity Gingerbread.
Mr Cameron insisted the Government could not remain "neutral" on the issue of stable family units, and had to send a "long-term signal".
"My view is that there is an importance of trying to say that commitment and relationships and marriage are good institutions. They are good things," he said. "We shouldn't be completely neutral about them as a society.
"I think the tax system and the benefits system as well as helping people in the here and now as it should, also does need to think about what other long-term signals that we are sending out as a society.
"We should be sending long-term signals that are about commitment and the things that encourage commitment. It doesn't disadvantage other people, it's just saying that this is a good institution that society should recognise in such a way."
However, Mr Cameron also stressed that a Conservative Government would not ignore the difficulties single parents faced.
"Lone parents will not be on their own under a Conservative Government," he said. "There is not and never will be a war on single parents."
Apart from defending his party's proposals for tax allowances to be transferable between married couples, Mr Cameron warned that society was ignoring the role of extended families.
He said houses and flats were often being built too small for people to enjoy a family meal, or for grandparents to stay if necessary.
There had been reports that Mr Cameron's aides requested that media access to the event be restricted for fear that he would get a rough ride.
After his speech, one woman single parent demanded: "You talk about tax and that you are looking at providing tax cuts for married couples.
"Do you not think that is discrimination against someone who pays their tax bill every month, and someone who is going to find that my friend down the road who has managed to find Mr Right gets away with paying less tax than I do, just because I am raising my children by myself?
"Also don't you think that's perpetuating the stereotype that actually a married couple is better than a single family?"
Another woman was greeted with applause when she warned that politicians were reinforcing false stereotypes about single parent families.
"What is the Conservative Party going to do to help organisations like Gingerbread to help smash down the images and stereotypes that we have of lone parents?"
Mr Cameron replied: "I think we all have a role in trying to make sure that we portray things in a more reasonable way and a less black and white way."
Gingerbread chief executive Fiona Weir said: "While we welcome his strong message about stigmatising, we would like to see them reflect that through policies.
"He risks making single parents feel like second-class families."
Financial Secretary to the Treasury Stephen Timms accused the Tories of being in an "utter mess" over marriage tax breaks.
"He (Mr Cameron) says he's deadly serious about marriage and intends to spend billions on a tax giveaway to wealthier married couples.
"Then he tries to deny to an organisation of single mums that he intends in any way to discriminate against them.
"The truth is, his marriage tax allowance is unfair and unfunded - it would cost £5 billion and give 13 times as much to those at the top than those at the bottom."