maybe it has something to do with something called equal rights? I dunno...just a shot in the dark...
marriage is not a right, it is an institution.
It used to be all about actually NOT needed to marry and have fun - all along the gay history thorough centuries, and suddenly, in the last 5 years it becomes a burning issue of such omnirelevancy that nothing trumps it - not the economy, not the poverty, not even the sacred race issue ( though it is pure American thing).
European countries have established civil unions more than 20 years ago - so the issue of benefits/inheritance has been resolved.And suddenly it becomes a necessity to sanctify the civil unions all the way up to marriage. Why? The answer is simple - to redefine the institute of marriage as the base cell of any society.