In the Friday evening leaders questions, Jeremy Corbyn described the marriage tax allowance - currently worth £250 to married couples - as a form of discrimination, which he would abolish*.
Is the tax on plastic bags discrimination against people who don't use paper bags, large pockets or cardboard boxes?
Is the tax on petrol and diesel discrimination against people who use cars rather than bicycles?
Is the tax on alcoholic drinks discrimination against people who prefer beer to lemonade?
Is a higher rate of tax on people earning £80,000 or above discrimination against the better paid?
Most of the taxes above are attempts to influence behaviour, and raise/redistribute money at the same time. And there is a very good case for influencing the behaviour of couples towards marriage. People who are married report higher satisfaction in the relationship than cohabitees, and cohabiting parents are 3x more likely than married couples to split up before their children reach 5. The high divorce rate comes a distant second to the breakup rate of cohabiting parents. The relative stability of marriage compared to the alternatives is clear, and consistent.
There are other factors - marriage is still the 'done thing' amongst the middle classes, and those with higher education, so there are cultural factors. Also, the financial barriers to marriage which come with the consumer add-ons now considered normal (dress, reception, gifts to the guests) make it prohibitively expensive, ruling it out for most people on low incomes. But despite this, married couples make a public commitment to stability and faithfulness, which when seen through is of huge benefit to the couple, to their children, and to society. So why shouldn't the tax system give this a little bit of encouragement? It's not discrimination, its encouraging beneficial behaviour.
*(Incidentally, the Labour manifesto doesn't refer to this policy, or at least any search for 'marriage' 'couples' 'tax' 'allowance' etc. doesn't find it, but if you've the time to read the whole thing and I'm wrong, then mea culpa. The Lib Dems have the same policy, and their manifesto has it in print).