This same-sex marriage
article contains a rambling, tin-foil hat, sexist, paranoiac screed, but he brings up an interesting and relevant sociological question.
If there is no provision that restricts same-sex marriage to gay people, then some percentage of people will start getting married for other reasons than pair-bonding. The question will be what percentage of the population will be paired as platonic couples; friends temporarily marrying friends for economic reasons, no biological mate stuff involved, and what will it do to the idea of marriage and the society at large? If it can be friend:friend with no reproductive or sexual component, whom may I theoretically marry? Marriage is currently about sexual access, but what if in future it had no sexual component from the get-go? If that is the new schema, then incestuous marriage is no longer incestuous, because marriage does not imply sex? I don't know. Maybe no one does until we get there.
Before anyone thinks I am siding with this guy who clearly has never ever met a lesbian, or possibly never met a woman, what I mean is, this is a broadening of the definition of marriage, marriage is one of the major cultural institutions, so it will have all kinds of interesting ripple effects that no one has even thought about.
I was thinking something similar listening to a radio show interviewing Colorado officials dealing with how to regulate and tax marijuana now that the feds claim they are no longer actively prosecuting the small-time growing or use of it. They are dealing with unknown unknowns in terms of how much to tax, how to keep it from going out of state, a whole bunch of practical concerns that nobody really thought of ahead of time.