When it comes to contributions-based benefits, treating people differently based on marriage is fraught with issues though. I may be wrong here, but isn't that why married women could pay less into NI than unmarried ones - as they were assumed to be supported by their husbands and inherited their pensions, which were paid as a couple? That has caused problems for those women down the line, particularly if they divorced.
Now the new state pension is not paid to widows there is no need to make patronising assumptions about who pays for what in households and give more to one person than another, or to assume that bills are paid as a unit rather than as individuals - they will be in some households but not in others.
Married men used to have a special tax allowance, as 'head of the household', too, so a husband and wife on the same salary would have different take-home pay, and married men took home more than single ones. Would that be seen as fair nowadays? I doubt that many women would be in support, and nor would cohabiting couples. Now that gay marriage is allowed, would both husbands be able to claim, whilst lesbians were denied?
As we are all taxed at the same rate as individuals nowadays, I can see no reason why household incomes should come into it. Also, it would mean HMRC and probably DWP linking the financial affairs of married couples, or people who are deemed to be in a household.
Giving the allowance to everyone, then taxing it back from those not entitled to it seems fair to me.