In order for one to suggest that there is no legitimate argument against gay marriage, you'd have to rationalize what it is that we in the west know better than those living in less-wealthy, less-industrialized societies outside our corner of the globe.
Most of those other societies incorporate substantial religious influence in their legislative and/or judicial processes. It doesn't require "rationalization" to demonstrate that this is harmful to human progress.
As an atheist, I agree, but asserting that the rationale behind prohibiting gay marriage is mostly religious isn't supported by evidence. More than a quarter of the world's population lives in countries that are majority-atheist without being any more supportive of gay marriage than those of us in western, more-religious countries.
EDIT: Apparently that's a little contentious -- only 47% self-declared atheist (http://redcresearch.ie/wp-content/uploads/2012/08/RED-C-pres...), but given that 9% of those surveyed did not respond to the question, the majority of respondents to this specific survey question did self-proclaim themselves as atheists.
Naw -- not really illegal; although it's probably politically complicated to be religious: "No state organ, public organization or individual may compel citizens to believe in, or not to believe in, any religion; nor may they discriminate against citizens because they do, or do not believe in religion." (Article 45 of their 1978 Constitution).
Even today -- decades after the state/social pressures against religious affiliation disappeared -- most former soviet states still have very large atheist populations.
Most of those other societies incorporate substantial religious influence in their legislative and/or judicial processes. It doesn't require "rationalization" to demonstrate that this is harmful to human progress.