1) Legal position: I'm a lawyer and marriage remains the most watertight protection for the family unit. Few cohabiting couples adequately protect themselves legally in the event that their relationship breaks down.
But the fact that some 'few' do (although I would like statistics, please, and indeed a definition of 'adequately') demonstrates that it is possible. And once again, we point to the fact that the laydee in question is really quite legally competent and presumably has, you know, other lawyer friends who can help out in those areas in which she does not personally specialise...
2) Two people can commit to raising a family together without being married. But, marrying someone is making a committment to that person, that you love, respect and value them and want to build your lives together. It's a way of making a statement, as are the vows that one makes.
That's purely your opinion. And in any case that is not the only reason people get married. They get married because they 'ought' to. They get married because someone else is putting pressure on them. They get married because they want a demonstration that their partner loves them, and think this is the best way.
3) Studies continue to show that the best environment in which to raise children is that where the parents are married (whether heterosexual or not). Cohabitees are statistically more likely to have their relationship fail than those who marry.
Which studies? How many and who conducted them? And the point about cohabitees is made above. (I would query, as well, the use of the term 'fail' - yes, some relationships, including ones that have produced children, finish. This is not in all cases a Bad Thing.)