Co-parenting with an ex is not for the faint of heart. But when your ex starts relying on you for more than just caring for the child you share, it can get really dicey, really fast. 

At least, that's what has happened to one man on Reddit, whose situation with his ex is quickly spinning out of control.

His ex is furious he won't babysit her kids — 5 of whom are not even his.

Deadbeat dads are everywhere, and fathers who call childrearing "babysitting" and think changing a kid's diaper once makes them a hero are probably even more common, at least if social media discourse is to be believed.

But there are lazy, uninvolved dads, and then there's this guy on Reddit who's being accused of being a selfish, negligent father because he won't help his ex out with childcare for her five other children, none of whom are his.

"I share a 12-year-old son with my ex," the dad wrote. They broke up when he was three months old and "got along fine just after" with "zero animosity" toward each other and an amicable co-parenting relationship. That has all changed in the past four years, however.

His ex has had 5 other children since their son and has repeatedly relied on him for childcare and financial help.

"Following our breakup, my ex has had five other children by five different men, and to the best of my knowledge, not one of the men is involved in the life of their child," he explained. She's raising the children on her own and has since moved to a different city.

"I have custody of our son, with her getting monthly visitation and more time in the summer," he added. Still, the reason he has custody is pretty bracing — "she was made homeless … after her former landlord sold the house she was renting," he wrote. "I was awarded temporary custody of our son, which turned to full custody once she moved."

But their troubles started long before that crisis. "The more kids she had, the more she would ask for me to 'help' with them," and the more he helped, the more she asked for — from "money for her kids' birthdays" and doctor's visits to "requests for me to take kids overnight or for me to babysit for her." 

He finally put his foot down, but she has not gotten the point.

His ex told him to 'man up' and that it was his responsibility to help her as the father of her first child.

Now that school is out, his ex has begun asking for help all over again — and he's flatly refused, especially given the circumstances.

"It's a three-hour drive from her to me, and I don't believe she's going to make that drive to and from here every day to pick up the kids, which means I would end up with her kids overnight," he explained. She also expects him to care for her kids for free. She's also pregnant with her sixth child.

His refusal turned into a fight in which she "laid a huge guilt trip" on him, telling him it would allow their son to see his siblings more often and that helping her was his responsibility. "She told me I need to man up and help her as the father of her first kid," he wrote.

People urged him to set and hold boundaries with his ex, given her increasingly chaotic choices and demands.

"Her inability to use birth control isn't your [expletive] problem," one Redditor bluntly replied to the dad. "It's not your responsibility to care for children you had no hand in conceiving. Keep that line in the sand, or you'll become more and more liable to kids that aren't yours."

The dad agreed, but in a testament to his empathy said he felt guilty and wished he could do more. That's admirable, but it's also incredibly dangerous — especially since he fears his ex's actual motive is to dump her kids on him permanently.

Divorce expert and life coach Dr. Karen Finn told us that when it comes to toxic exes, boundaries are the most important key to co-parenting. She suggests limiting contact by only responding to requests pertaining to your kids, maintaining a business-like relationship, and, if need be, obtaining a court order to keep your toxic ex in line.

That seems like good advice here — especially given his worries about her abandoning her kids with him. "Do not under any circumstances ever accept responsibility for any of her other kids for any length of time. Never," one user cautioned.

"You need to quit engaging in conversations that involve anyone other than the child you conceived," another suggested. "Every time you give reasons why you can’t parent the other kids, every time you argue, it gives her a crack in the door to push through."

Bottom line, she's manipulating him, and it's abusive. His desire to help is admirable, but some people simply cannot be trusted, and others' choices are not anyone's responsibility but their own — no matter how hard they may try to put them on us.