There's a write-up on the Common Health blog, and it seems they are considered unhealthy because of all the unrealistic imagery and situations they contain. Not unlike magazines airbrushing the already almost-flawless supermodels, romance novels create a nearly-impossible fantasy world. If romance readers aren't diligent about separating fictional fantasy from reality, their expectations can get skewed, which can lead to unfulfillment, disappointment, and depression.
The article also referred to non-consensual sex, and the excitement of women being "taken" by dominating alpha-males. And that safe-sex is continually portrayed as unromantic. It seems that most of this would be counteracted by simple common sense (I watched a lot of Bugs Bunny growing up, but never tried to walk off a cliff or drop an anvil on someone), but their findings indicated that there is a correlation between frequent reading of romance novels and a disregard for healthy sexual practices.
Which is especially worrying in the ebook era, as the introduction of ereaders has increased the popularity of romance novels. Anecdotally, they're less embarrassing to read now that ereaders allow them to be read in public without anyone being able to see what your reading by the cover - although to be totally hidden, readers also need to keep their heaving bosoms in check.
Whenever I hear of something like this, my first reaction is for the library to try to somehow protect patrons from it. But you cannot protect people from themselves, and it's not really the library's place to restrict what people read - we can provide information, but they need to make their own decisions.
But wow, it would be funny if we had to ration patrons to no more than two romance novels a month - I'm sure our circ stats would take a hit.