'I tolerated my boyfriend's friendship with his ex. Until one request crossed the line.'

For five years, all was well with Lauren* and John*. They were in a "loving, supportive, and easy relationship".

Both had children from previous relationships. John, 40, looked after his daughter 90 per cent of the time.

"My boyfriend and his ex wife, we will call her Susie, have a very positive and friendly relationship," Lauren, 33, wrote on the popular Reddit thread Am I the Asshole.

Watch: Co-parenting advice from an expert. Post continues after video.

But, she admitted, it "took some getting used to" when Susie would enter her boyfriend's house unexpectedly and "make herself comfortable". Often with friends in tow.

"My boyfriend who is very non-confrontational, set no boundaries as it allowed him to parent his daughter consistently," Lauren said.

"Susie has not demonstrated a great interest in the day-to-day parenting role, so he likes to keep the peace so his daughter has the stability she needs."

The issue began six months ago, when Susie exited her long-term relationship.

"Suddenly [Susie's] presence has increased significantly," Lauren said. "She will invite herself for supper and along on activities we've planned with the girls.

"She will call my boyfriend numerous times a day, unrelated to their daughter. He won't answer when we are together, but it's still aggravating because she'll then call their daughter to get his attention."

While Lauren said she has "stayed in (her) lane" for the most part, it was the latest incident that changed everything.

"[Susie] invited herself along on a holiday we planned for our girls, claiming it's a once in a lifetime experience," the 33-year-old user wrote.

"It's not, it's Disney, and she took her daughter there two years ago when she was still with her partner."

Lauren told John that he needed to set a boundary and tell Susie she couldn't join the trip.

But the 40-year-old man said he was "uncomfortable about rocking the boat and her seeking more time with their daughter, as he doesn't feel [Susie] is stable enough to provide consistency or stability".

Regardless, Lauren was not interested in spending her vacation with John's ex-wife.

"So I told him to pick. Am I the asshole, or are my feelings valid?" she asked Reddit.

The replies came in thick and fast, with the majority taking Lauren's side in the argument.

"He's made it clear that he is completely unwilling to set boundaries with his ex, either now or in the future, and it sounds like his ex has picked up on that," one wrote.

"Now your daughter's holiday is going to be impacted by his unwillingness to set boundaries. This will continue to happen until you end things with him because, as he's made clear, he's not interested in setting boundaries with his ex. So that's never going to happen."

Another agreed, adding: "She is literally going to be the fifth wheel while they play happy family at Disney. Hell no.

"I would have said something ages ago but then again I think anything beyond cordial with the exes is absolutely a hard no. [Lauren] let this woman trample over her boundaries and now things are out of control."

A third pointed out that the boyfriend was "walking on eggshells because he's afraid of his ex".

"He needs to either get back with ex or go solo until his daughter is 18 if that's how he chooses to handle things.

"It's not fair to you to have his ex in your relationship. Don't give him an ultimatum. Do this for yourself. Leave. It's not like there's a shortage of guys out there."

Someone even suggested the boyfriend was "spineless", claiming he was setting a bad example for his child.

"I get that he doesn't want to cause problems for the sake of his kid, but avoiding them will make them snow ball into a bigger problem," they wrote.

"And it's already starting to do that. Susie will keep pushing. Your boyfriend is allowed to have quality time with his kid with out his ex asserting her self into it every time. The ultimatum might be the push he needs. The ex is running your relationship. Not him or you."

*Lauren, John, and Susie's names have been changed for the purpose of this article.