We remain together because we both want to, not because the law or society says we should.
I'm always interested when people list this as a reason not to marry. Obviously do whatever you want, but do you think the law says a married couple should remain together bonnie? If so, how do you reconcile this with the existence of divorce?
OP, if you're looking at it from a purely legal perspective, what you 'should' do is going to vary depending on your circumstances and what you want. I would always tell anyone to get a will and also life insurance. It's particularly important if you have a partner you're not married to and want to benefit from your estate. This is because the intestacy provisions, ie the rules used to distribute the assets of someone who doesn't have a will, don't cover unmarried partners except in very limited circumstances.
But there are other things that depend more on your particular views. For example, you and your partner can of course make wills naming each other, but you can also change it without the other knowing, and they're unlikely to be able to challenge it. Whereas they could if you were married. You might see that as a good thing or a bad thing, that's personal, but the thing to remember is there's nothing you can do about it either way.
I would also thrash out financial issues before you TTC. For example, will either or both of you be reducing your hours for childcare? That's not something an unmarried partner of either sex should do without careful thought and/or their own assets. Because if you split up, there's no possibility of spousal maintenance and getting the non-earning contribution recognised in division of assets is infinitely more difficult. You will also need to think about who funds any maternity leave.
Also do you rent or own? If you own, how is it split? This is particularly important with unmarried couples.