As far as gays go with this default stuff in case of marriage, it's quite simple. Marriage is an age old institution with the purpose of creating stable families capable to having and raising kids in a stable environment. Since this has a positive effect on society, the government accomadates it in order to encourage people to do it, or at least remove some of the obstacles. Gays, however, cannot provide any of these effects. A gay couple cannot reproduce, no matter what the Massachussetts birth certificates say, and it has been statistically shown that children who do not have both a mother and a father are far less likely to succeed in the real world. Therefore, those obstacles are not removed for them, and they have to fill out inheritance and power of attorney paperwork instead of a simpler marriage liscense.
As with most things, there is a practical, business-like reason for not having gay marriage.