They've been together long enough that everyone assumes they're married. They don't bother to correct the misconception, anymore. There's not much point - and it's not like either of them will find anyone else.
It's not a real marriage, though; not the way a marriage should be. It's not that they don't love each other, but it's not that kind of love. It's a hollow emotion, born out of loss and loneliness and having loved someone that couldn't love back. It's the sort of love that comes from a life filled with too much death, from growing hard and only being able to let the person that's grown hard with you know there might be anything left that's soft.
Faye likes it best when Jet takes her from behind, because then she doesn't have to look him in the face while she's thinking about someone else. She can't decide if she feels any guilt for that or not - she's fairly certain he knows she's doing it, and suspects he doesn't want to look her in the face, either.
She's fairly certain he's thinking about someone else, anyway. It's more that she wonders whether it's a woman, or whether the name that doesn't make it to his lips is Spike's, too.