Memorable and moving encounters with constituents often inform MPs' contributions to debates. I frequently draw upon them. During the 2010 general election campaign, I knocked on the door of a single mother who had recently left her husband because he had been violent towards her. She and her two sons were living with her mum. The father was making no financial contribution so she was working hard and her mother was helping out with childcare. This mum was angry and upset. She told me how hard she and her mother were trying to give her children a good life and to bring them up to be respectful and to work hard at school. She had heard on the news that morning that the Tories were promising a tax cut for married couples. She wanted to know what she had done wrong, why she didn't deserve the same break and if David Cameron expected or wanted her to stay in that relationship.

I remembered that mother yesterday as the House of Commons debated Labour's amendment to the Married Couple's Tax Allowance. As I listened to the debate, I felt like I had travelled back in time to the 1980s when I was a school governor in Oxfordshire. School boards had just been given the power to establish sexuality education policy. I suppose it was inevitable that in a Church of England village primary school all those years ago, this led to a vigorous debate about relationships and families. I didn't win the argument to adopt a policy which saw homosexual relationships and different kinds of family as equal. With the abolition of Section 28, equalisation of the age of consent, and same sex marriage legislation, I thought that times had changed. My argument had in time been won.

However, as we debated the tax allowance, I lost the argument all over again. I heard the same kind of language and views that I had heard in that school board almost 30 years ago; I realise that the Tories believe in the deserving and undeserving poor, but it is now clear that they also believe in the deserving and undeserving family

Speaker after speaker on the government benches spoke about families and how this policy was designed to help families. That is not what this policy does. Unless you are truly of a different time, you recognise that families come in different forms. Even this tax cut offers equal treatment to same sex marriages and civil partnerships. It was a strange experience to hear Tories who had voted against equal marriage standing up for tax breaks for couples they didn't think had the right to marry.

That wasn't the only illogical case advanced by Tory MPs. They declared that married parents of poorer families are more likely to stay together. I am sorry if this sounds flippant, but that doesn't sound like an argument for giving them more money. We had Tory after Tory proclaiming that children, adults and even grandparents have better outcomes when marriage works. They confused correlations with causation and failed to make the link as to why these outcomes of marriage should result in a tax cut, Surely these people feel blessed enough and would rather, in a time of austerity, that the state used its resources to help those in greater need.

There was much talk of families in this debate - Tim Loughton MP referred to what "these families deserve". This is just one example of the government being careless and potentially offensive in the way they framed this debate. He and his colleagues were not speaking about helping families; they were talking about helping married couples with children. Unless they believe theirs is the only definition of a family. Just as oranges are not the only fruit, married couples with children are not the only families. Even then, their offer will only reach one third of married couples.

That is not my real problem with this tax policy, nor is the objection raised by many that widows, widowers, people who have left abusive relationships, kinship carers and many others would miss out. I don't want to talk about deserving and undeserving single parent families either. That is the politics of division - the politics of this Tory/Lib Dem government. It is none of my business if someone asks for my help, what their relationship status is, neither is it the business of government or HMRC. A parent raising a family on their own deserves our support if they want and need it. That should not be conditional on whether they were the "guilty" party in the breakup of a relationship, or if they were ever in a relationship, because we all have a stake in helping every family raise healthy, happy and useful citizens.

I am going to do it again, what young people describe as "oversharing". When my marriage of 23 years ended, it was a traumatic and upsetting experience. In many ways it was like bereavement, with a destructive helping of shame and guilt and, for afters, the strain of trying to support and reassure our four children. A tax break wouldn't have saved my marriage and when I was giving myself a hard time for having failed at the most important relationship of my adult life, I didn't need a letter from HMRC telling me that my tax code was going to change.

I, and their father, spent more than two decades doing our best to provide for my four children, to guide and support them in the hope they would become useful and caring members of society. My sons and daughter are now making families of their own. It makes me beyond happy to see that having children is something they want to do and how well they are doing it. I do my best not to interfere in their lives now they are grown, to accept them for the people they have become and the choices they have made. It is none of my business if they have chosen to marry or not to marry and it certainly is none of the business of this government.

Fiona O'Donnell is Labour MP for East Lothian