Not only are they empty statistics, they're falsely correlative. MA has some of the toughest divorce laws in the country. They always have. It's harder in MA than in most states to actually dissolve a marriage, which thus means, fewer people actually do so. Showing the rate of divorce without also showing the rate of marriage doesn't prove anything. I remember the NYT reporting on MA's low divorce rate and gay marriage being a mixed blessing for LGBTs two years ago (seen
here) that mentioned some of the same points. I'm not aware of PA's divorce laws (though I'd guess they're probably as stringent given laws usually don't vary much within certain regions) or DC's divorce laws and rate of marriage, but just mentioning one type of statistic isn't a great argument.
I'd say when people say gay marriage will ruin the institution of marriage, they're not necessarily talking about divorce. To many social conservatives, divorce is a separate issue affecting marriage, and many would likely tell you it's a bigger threat than gay marriage. Social conservatives see marriage as a unique union between a man and a woman for the primary purposes of having and rearing children and perpetuating that ideal in society. They think normalizing gay marriage will further pervert the purpose of marriage in society. I don't think anyone in their right mind would say it's the
cause of that perversion, but it could be a definite factor (i.e. no-fault divorce, which many social conservatives opposed in the 70s) which is why they so adamantly oppose it.