To apply joint taxation to married couples only would be discriminatory. But the idea of the Inland Revenue administering a cohabitation rule is a nightmare. It turns into an administrative absurdity once the increasing fluidity of relationships is taken into account.
To point out that means- tested benefits are assessed on a family basis does not justify a return to joint taxation; instead, we should be exploring how to move towards individually-based benefits, as the European Commission has recently suggested. If we are worried about the widening gap between rich and poor, there are plenty of ways to increase redistribution from the better-off - whether married, cohabiting or single - without abandoning independent taxation.
It would be a mistake to tax child benefit: it would be rather odd to get only families with children to pay for an increase in the one benefit specifically for families with children. In addition, under joint taxation, many husbands could be paying tax on the child benefit received by their wives. The resentment this caused could result in calls for the reintroduction of child tax allowances, paid via the main earner - usually the man. And then we really would be back where we started.