I would fill in the form as "married" only if the country recognized my marriage, which, at the moment (according to Wikipedia, which reflects the news, which I follow closely) means Argentina, Belgium, Canada, Iceland, the Netherlands, Norway, Portugal, South Africa, Spain, or Sweden, all of which allow same-sex marriages, as well as Israel, which recognizes other jurisdictions' same-sex marriages.

It would gall me to tick "single" when I am not, but why create an issue when you know the visa-issuing authority doesn't recognize the marriage?