Some parts of the United States recognize same sex marriage and have laws that demand equal protection over the law for those spouses. After Massachusetts in 2004, many other states have legalized the practice and consequently extended the benefits that companies offer their employees' families to these additional members.
There are many different factors to consider when it comes to offering these benefits. Areas that recognize the practice are likely going to provide them with little thought or discussion. Places that currently feature hotly contested arguments on the matter will likely stay on the side of conservatism. Locales that have blocked it or not even considered it often won't even broach the subject.
However, some places that don't feature legal same-sex marriage might want to make its attendant benefits available nonetheless. For example, Baltimore County, Maryland, government workers were recently extended the right to receive family employee benefit coverage. While same-sex marriage isn't sanctioned in Maryland, there are probably enough employees there that either want full rights or who support their colleagues who do. Consult with an employee benefit specialist to understand what the best options available are.