@ marantha I know this is more relevant to the Camerons and Osbornes of the world, but if you die leaving property worth over the £325,000 nil band (so quite likely if you die having paid off your mortgage and live in the South East), you don't pay Inheritance Tax on anything you leave to your spouse.
Also, if your spouse dies and they haven't used their nil band, you get to use it, so have a nil band of £350,000.
Marriage benefits relating to income tax may have disappeared, but they are alive and well in the rest of the tax system - including no capital gains tax on transfers between husband and wife.
Re: rights of cohabiting couples, this article is a bit old, but I don't think much has changed since it was written.
news.bbc.co.uk/1/hi/magazine/3746599.stm
Living with somebody and fathering/bearing their children doesn't give you much status in law. Spending 15 minutes in a registry office does.
Assuming, you want to have a long relationship with your co-parent, I think there are tons of incentives to get married.