Here in Belgium, in tax law most differences between people who are married or just living together have been worked away. There's still some work to do (and certainly so in the social benefits department), but it's not impossible for the legislator to get rid of all differential treatments of people just living together and people who are married. I don't think one group should have any advantage over the other regarding tax law and social benefits. I think the circumstance of having a household together or not should be decisive. Besides, plenty of people are married and don't live together, so why should they be treated as if they are living together. Really, your marital status should be completely irrelevant when it comes to taxes and social benefits.
The legal framework should only have consequences on the private (civil? or is that the same over there; here it's "burgerrechtelijk") law level; in other words: only consequences for those in the legal framework. Marriage/living together is a strictly private affair, imo. Providing a legal framework that people can chose for seems like a no-brainer to me; living together has consequences and the state has a responsibility to at least provide some sort of framework (optional of course) to deal with those consequences (especially with the situation once the living together has ended). But that should be it.
Anyway, whatever the legal framework, getting a prenuptial is always a good idea.
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