Some people think state-recognized marriage is a special privileged legal status for the purpose of having and raising kids. Ie, that the focus actually isn't on the adults who are the ones to get married. In which case, couples who know they can't possibly have kids are trying to con the state out of the benefits without accepting the associated responsibility. I don't know what faction of people with this view take is to it's logical conclusion regarding for example infertility, but I'd be very surprised if there weren't at least a few and not completely shocked if it was a majority.