There is anger on the streets of Oxford and Abingdon. And rightly so. David Cameron's idea to bribe people with £3 a week to stay or get married is archaic. Not everybody lives in couples where one partner goes out to work and the other stays at home. People are outraged at a man who has the audacity to use the tax system to judge their decisions about their personal lives. Or worse, decisions that have been made for them.

A new campaign called Don't Judge My Family has captured public anger on this issue. It asks people to upload photos of themselves and their families to celebrate the diversity of family life in the UK.

The case studies speak for themselves. Jim, a single dad whose wife walked out on him and his three kids says: "My wife had an affair and walked out on me and Lilly. As if I don't feel bad enough already about it. Especially as I'm now living on peanuts, trying to make ends meet. Talk about kicking a man when he's down." Another, a mother and teacher, who has just gone back to work to earn a bit of extra cash says: "I have to work part time to be able to afford the basics for the boys. But does that mean I'm a bad mum? Quite the opposite, I think."

The Conservatives have once again put political posturing over evidence; a deplorable trait in anyone standing for public office and one to which the Liberal Democrats have pledged not to fall prey.

First, there is no evidence that marriage is better for kids than any other stable relationship. New research this week from the Institute for Fiscal Studies examined a representative sample of data on 10,000 children. Although it found that children born to married parents were likely to do better by age three and age five, once the study had controlled for parental socioeconomic factors (such as education and wealth) and whether the pregnancy was planned or not, the differences disappeared. Kids tend to do better in wealthy, well-educated families, where pregnancy was planned. And these families tend to be married. Being married isn't the causal factor.

Second, even if marriage were good for kids, there is no evidence that the marriage tax allowance would encourage people to get or stay married. On the one hand, there are all those people who would become or stay married regardless of whether they received £3 a week: people who marry for love. On the other hand, there are those people who would split up whether or not they received £3 a week. Try telling a woman fleeing domestic violence or going through a painful divorce that £3 a week will make it all worthwhile.

Third, even if marriage were good for kids and the marriage tax allowance did encourage people to get or stay married, the policy won't reward the people the Conservatives want to reward. The IFS shows that it will only reward a third of married couples. Two-fifths are pensioners, many of whom have been married for donkeys' years. Only 35% of the families who gain from the policy have children, and only 17% have children under five.

And it is meaningless to point out, as some Conservatives do, that couples who marry are more likely to stay together. To me, this seems like a self-selecting sample – surely people get married because they are committed to each other. They would have that commitment whether married or not.

Giving children a good start in life is too important an issue to replace evidence-based policy making with crude political posturing. Sign up to the campaign at Don't Judge My Family.