It's not just inheritance tax, it's other things that are also discretionary if your partner dies and you are not married. Like death in service benefit from his workplace - if you are not married the administrators of the scheme can decide they don't want to give it to you. if you are married, they are obliged to, under the terms of these schemes. And you won't get widowed parents allowance either, it's based on your spouse's national insurance contributions. You will need these things and more if you have kids and your other half dies.
Believe me, me and my kids are the worst case scenario, as my husband died when they were young, still needing childcare when I worked, and those sums don't add up very well even with every bit of financial help, never mind without it. Even though I had always worked, to sort everything out alone when you are fucking devastated is beyond hard, without further financial complications or having to leave your family home. It was the first thing my kids asked me: "Are we going to be able to stay here?"
It has been extremely hard anyway, but being able to stay in the same house, go to the same school, be near their friends was absolutely crucial for my kids (and for me, really). I work full time, they are now teenagers and we are all still here and breathing in and out, I have been a good little stoical bunny, but it is still, as any widow or widower will tell you, a big fat crock of shit.
Actually I have a lot of sympathy with those who are saying Why should we get married, load of patriacal (sp sorry) crap. We didn't get married till we had popped out a couple children, but I have reason to be thankful that we did now. It doesn't mean that I am with hindsight some sort of surrendered wife with no anger about society's many misogynies.
I suppose this is going to come over as rather melodramatic and alarmist and of course it is a minority of partners who die young but it is something to mull over. Yes I suppose the state does blackmail you into it - but I wouldn't have liked to have had to cope with a lot of financial arguments and traumas as well as everything else.