Second answer: in a sense, they don't really permit single members past a certain age. While they're still teenagers, churches do their utmost to attempt to tamp down those adolescent urges. After that, those that don't get married right away are treated so thoroughly as unwelcome (unless they pretend to have no sexual desires or any other sensual desires) that they drive them all off. Those that do return do so after they get married and have a child out of some sense that they better make sure their kids get religion.
Churches are, in my experience having gone to dozens of them, predominantly populated by couples with children, grandfathers dominated by grandmothers, and widows, the last group doing their best to pretend that they just love being lonely and enforcing that standard on their fellow widows while providing moral support to the older women who still have husbands, especially husbands who still have a sex drive.
When a single middle-aged man shows up at church, no kidding, there is a fury of casseroles and female attention, but if the man doesn't rather quickly attach himself to one of the unattached middle-aged women, he tends to start being treated as if he may be some kind of moral threat to the morale of the church.
But maybe these things just represent my experiences.