Marriage was a religious ceremony that was turned into a legal one and then the government incentivized it (in the form of tax breaks) to encourage the behavior as marriage does have societal benefits such as having offspring.
Just another example of how government shouldn't really be in our lives depsite good intentions. I agree with the above poster, take a step back and look at how embedded what we do on a daily basis and why is a function of government.
I'm not some crazy antigovernment tin foil hat wearer but it does bother me how much of our behavior is dictated by it.
Who cares if someone wants to "marry" a bird? Does it impact you and what life you live? If there are no "incentives" or tax breaks or whatnot it becomes merely some random ceremony, all that's happening now is you have some people dictating what others cannot do. How free is our society, really?
Marriage used to occur much earlier in life for farm production reasons. And the "women" were very young and had kids from the get go, what would be considered unethical and illegal today. So keep the age of consent, if it truly is an issue. As it is most kids even 20 years ago were having sex in the 14-16 range.