Where I'm from - Germany - marriage has been a contract under civil law for about 120 years, thanks to Bismarck. Religious ceremonies may be performed (after the civil ceremony), but are legally irrelevant. Works fine for everyone. Yes, the state governs the legal contract (which is fine with German social tradition), but there is room for religious people to add what they deem necessary.
IMHO that's a good solution: What matters is the legal contract, as religious considerations are irrelevant to me. I personally see marriage not as a moral or spiritual matter, but a contract that is supposed to create an environment which is supportive of building a family - by granting rights and entitlements. Which is to say that I would prefer most of those entitlements to be connected rather to biological and adoptive parenting (yes, that includes gay couples).
With regards to an inner meaning of marriage or any relationship for that matter - no legal or religious institution can make people value it - it has to be filled with meaning by its participants. All the institution can do is not make it harder for people to do so by attaching prescriptive moral obligations.
Institutions will always be informed by social reality, however traditionalist or progressive one might want to be - effectively people will make individual choices and use the legal tools available to make the system fit their choices (case in point: the legal attempts to outmaneuevering by both the pro- and anti-gay marriage camps). That's why gay marriage has been moving forward in many societies, polyamory hasn't and for the foreseeable future won't. That's also why patchwork families are on the rise and clinging to the (relatively new and imho dysfunctional) model of the nuclear family really isn't working. What matters is what makes things work for people - and their (biological or adoptive/raised) children.