Wondering how to fill out your Census form? From an article on gayrights.change.org:
If you are in a relationship and you live together ... There are 16 choices, but the two that concern you are "husband or wife" and "unmarried partner."
This is the important part: You do not answer based on the actual legal status of your relationship, you answer based on how you personally categorize your relationship. If you are legally married, you will probably mark down “husband or wife,” though if you are in a civil union or domestic partnership, yet you still feel married, than you should also mark “husband or wife.” If there is no legal recognition of your relationship where you live, but you still consider yourself married, it’s also important that you mark “husband or wife.”
If this box doesn’t accurately represent your relationship, you have the option of the “unmarried partner” box. If this sounds like a vague catch-all, that’s because it is. The Guide to the American Community Survey says “An ‘Unmarried partner,’ also known as a domestic partner, is a person who shares a close personal relationship with Person 1.” If this better describes your relationship, than please mark the “unmarried partner” box.
You should also take a look at Queer the Census.