Being on the same page about whether or not to have kids is an essential part of a relationship. But what happens when one partner changes their mind? 

A man on Reddit is currently dealing with this situation, and his own history is making it extra messy.

He didn't tell his girlfriend he couldn't have kids because she wanted to stay child-free.

Being at odds about kids is such a huge issue it's often the death knell for a marriage. A recent survey showed that differing views on parenting were among the biggest causes of a split, and more than half of those asked felt their marriage could have been saved if they'd had a clearer understanding of their partner's views and values before getting hitched.

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There's no doubt about it: Having this stuff out in the open ahead of time is vital. But for this man on Reddit, it ultimately didn't matter — or so he thought, anyway.

He and his girlfriend both didn't want to have kids together, so he never thought to mention his infertility.

The man wrote in his post that conceiving his 11-year-old daughter was something of a miracle. The pregnancy was an unplanned surprise, but they didn't learn just how much of a surprise until they tried to have a second child and found out that he had a very, very low sperm count.

"I am lucky to have had one kid in the first place," he wrote. "I tried the treatments and it did not help, the count stayed low." 

This contributed to their subsequent divorce, which quickly turned into a lengthy and ugly custody battle that lasted two whole years. So it was probably something of a relief when he met a new woman who wasn't interested in child-rearing.

"She was great with my kid," he shared, but was very clear "right at the start" that while she likes kids and enjoys spending time with his daughter, "she would never want any kids of her own…so I should take that into consideration." 

That was fine with him. He told her he didn't want any more kids anyway, especially after his dramatic custody fight. Here's the thing, he didn't tell his girlfriend he couldn't have kids, thinking it wasn't relevant. But then everything changed.

Two years into their relationship, she's rethinking her child-free stance and furious he didn't tell her about his fertility issues.

"Now two years later we already live together, and she suddenly realized that she wants to have a kid." This was of course a huge surprise to him, "and I told her that there are two problems: I don't want more kids and I can't have kids." 

She at first thought his response was a reaction to his traumatic custody battle, but then he told her there was "a medical problem" in the mix. She was understandably furious that he kept his fertility issues from her, which has left him confused.

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"I did not think that it was something important as she did not want any in the first place," he wrote. And it's been made even more difficult by the fact that he is still firm in his desire to not add any more kids to the mix.

"I like my life as it is today," he said, " having a baby would mean starting it all over and we would have two kids with totally different needs."

Most people agreed that he didn't do anything wrong. But experts say even in situations like his, transparency is key. 

It's hard to argue that this guy did anything morally or ethically wrong. His girlfriend suddenly changed the rules of engagement. And as the man said himself, his fertility issues ultimately didn't matter, "she has to find someone else as even if I was fertile, I still don't want [another kid]."

That said, the drama now ensuing could have been avoided if he'd been upfront about his situation just for good measure — and many on Reddit felt like this was the ultimate takeaway.

One Redditor who agreed he'd done nothing wrong nonetheless added, "I understand her being upset that this has happened, and in the future, it might be a good idea for you to let potential partners know up front when you have that conversation."

Experts agree. Dr. Miriam Torres Brinkman, a marriage and family therapist, shared with us 100 questions all couples should ask each other before marriage. A full dozen of them are about having kids — including discussing what will happen if one of you changes your mind on the subject down the road. Not having this all hashed out before making a commitment, she warned, can lead to the couple "facing deeply serious dilemmas," because of course, these are "not easy compromise[s] to make."

Better safe than sorry, even if it seems like it doesn't matter at the time.

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John Sundholm is a news and entertainment writer who covers pop culture, social justice and human interest topics.