I actually waffled when writing that too! I refer to my husband as such because he is my husband. Our girlfriend does not wish to be married. She has stated many times that she isn't interested in marriage and never planned to marry (even before meeting us). Despite her saying this I do actually think of her (in my head at least) as my wife. I don't say that out loud because I don't want to push her or upset her. In the same way, I think of him as our husband. He has also referred to us as his 'wives', but I'm not sure if he's said that in front of her.
I think her opposition to marriage stems from the fact that she knows she doesn't have the legal right to marry the people she loves. Prior to dating us she dated mostly women, so she wouldn't have married them either, at least not in most states.
I find pronouns inadequate to describe the dynamics of my triad. It always comes off like I am leaving someone out.