I don't think anyone is unreasonable to have kids before marriage.
I personally didn't want to take on the huge commitment of kids alone, so taking on the lesser commitment of marriage first made sense to me, but that's a personal choice I think. (At the other end of the spectrum I know people who have deliberately and happily had kids without any father figure).
I knew someone whos partner died suddenly, leaving her in a complete mess, no bereavement allowance, the banks etc obviously would even let her see her partners financial details. Took ages and hardship before it was sorted. I always remember her saying 'marriage is just a bit of paper - but if the worst happens it can be a really important peice of paper!'.
I think people who get married first are sensible. I think people who decide marriage isn't for them and take the time and effort to arrange financial matters, joint accounts, wills, insurance etc, so if the unexpected happens (death, divorce, etc) they and their kids are protected are equally sensible.
I think people who don't think about the possible problems, or who intend to get married for the legal protection for them and the kids but then don't because they want a big wedding are frankly a bit stupid and irresponsible.
You can go to a registry office get married (about £150 round here), you don't have to change your name or tell anyone. Then throw the big white dress party (aka wedding) a few years down the line. Or you can pay a bit more and set up your own legal protections (wills, joint finances etc) as mentioned above. But having children is a big responsibility and you owe it to them to get yourselves sorted one way or other before they arrive.