Men are obliged to pay child maintenance regardless of martial status.
TBH rather than a lot of stuff people use as criteria to vote for candidates in elections, i think I'd go for (and i am in no way affected by this at all, my children are grown up and I've been married since forever) making paying CM a legally enforcable debt that leads to garnishing of wages and seizure of assets pretty darned quickly if there are arrears. Are any parties offering that?
Anyone can change most of these unilaterally at any time.
That is what would worry me, tbh. At least you can't divorce someone without them knowing, so (little as they may be) those legal protections are there. I think there is a way to kind of link wills, isn't there? Also wouldn't there be issues if your DP left you and your children, then married someone else and had a family, then died without leaving a will? Where i am you can't disinherit your children (well, you can but it is not easy) so that scenario doesn't leave the first family children out, but in UK it's different, i think? Also the other way round. If you marry then separate but don't divorce, so you can't marry your DP, if you die, that makes a problem for your DP against your still-spouse.
And while Church (other religions are available) marriage is pretty much still based on patriarchy (you can drop "obey" from the vows? great) a civil marriage is still pretty much a contract between two people. (although the civil service, IIRC, talks about lifelong commitment through thick and thin)
People need to know all these things. (not just those with children)