In reply to Anonymous:
Personally, I don't think it's important to be married before you have kids, or at all, if you don't want to be. The nature of the relationship is what's important
It *IS* convenient legally, since it ticks a lot of "boxes" such as inheritance, assignment of pension rights, property rights, etc, all in one simple document. We've had the argument several times on here about whether marriage is an outdated institution, with one person in particular arguing strongly that it is outdated. However, despite my first comment, I believe that if you want the state to acknowledge your status as a couple, this is by far the eaiest way to notify it!
Without marriage (and this applies whether or not you have kids) then, in order for both partners' wishes to be "known" by the state in case of problems (whether splitting up, death, major illness, guardianship issues of kids, etc) then separate legal documents would need to be set out in each case to cover eventualities that are automatic in the case of marriage.
It's all very well to say "we have a wonderful relationship, why spoil it by making a legal issue of property rights, inheritance, etc" but I know too many people who, when things have gone wrong, have ended up in a mess because of their legaly ambiguous state.
One of the most distressing was a couple I knew years ago where the bloke's parents hated the girl he was living with. The bloke died at work, abroad. Because they'd never got around to formalising anything (no wills, no transfer of the house to both names, etc), his parents were treated as next of kin, and *they* got his life insurance payout from his employer, the house, and everything. This was in direct contrast to what he'd have wanted to happen.