The phrase "Man Up" originated among men telling boys to grow up and start pulling their weight. Its variations included "cowboy up" and "sack up". Translated, it was

Look, I know you're hurting and it sucks and whatever else is going on, but I don't give a shit and neither does anyone else. There's shit to do, it needs done now, and we need your help. We need every swinging dick we can get out there getting the work done. So get your ass up, shake off whatever's ailing you, and get out here and help us get this shit done.

Somewhere along the line, probably in churches, women, and, well, everyone else, appropriated the "Man Up" exhortation. And, of course, changed its meaning. So, now, today in 21st century America, "Man Up" means:

You all need to get married now. There are women out here who need husbands and plow horses and providers, and you need to let one of them pick you, make you marry them, and start paying their bills. These women have bastards that need money trees "fathers". So you need to get in here, step up, get your wallets out, and start taking care of these women's bills and financial needs. You need to shut up and get over here and marry one of these sluts church girls that want husbands.