Would you be opposed to pregnant women taking this drug?

There is no legitimate argument against gay marriage.

So, what makes an argument legitimate? 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.

Using that as a rulestick, no one will be negatively impacted by homosexuals getting married. Studies have been done, plenty of places around the globe have tried it with no negative consequences. The courts agree. There's no government interest in preventing gay marriage.

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.

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.

Even today -- decades after the state/social pressures against religious affiliation disappeared -- most former soviet states still have very large atheist populations.