A girlfriend (platonic) who is the type you'd like to marry, only there's no chance of sex. Can be used by one girl about another girl, if the girls aren't lesbians.
Girl 1: OMG, will you marry me? You're so perfect for me.
Girl 2: I can't, you don't have a penis.
Girl 1: That's ok, you can be my wife without benefits (wwb).
A guy can also use this term about a good female friend who he shares his life with but isn't attracted to physically - i.e. there's no chemistry.
Guy: I tell you everything.
Girl: I know, we should get married.
Guy: It's almost like we are. You're basically my wife without benefits.
Also derogatory for a sad situation where a couple wants to stay together, for the children, and also for friendship love, but the steam has gone out of the marriage, or the wife is holding back to punish the guy, or the woman can't sustain sexual interest.
Guy 1: How are things going the wife?
Guy 2: They're alright.
Guy 1: Just alright, eh? You guys appeared to be getting along well at the party last night.
Guy 2: Ya, we get along. Things are ok. But I'm not sure how much longer I can last in this situation. I didn't sign for a wife without benefits.
Also used where a wife won't indulge in a guy's minor fetish, like oral or spanking.
Girl on net: You married?
Guy: Well I have a wife without benefits.
Girl: You don't have sex?
Guy: Only missionary.
Girl: And that's not enough for you?
Girl: Well, why don't you get divorced and look for someone new?
Guy: It's complicated.
Girl: Well, I'm glad you admitted it, but I'm not really into married guys. Too bad, because I do enjoy benefits...