Seth Rogen explains his theory why Katherine Heigl ended up hating 'sexist' movie Knocked Up

Seth Rogen has explained his theory as to why Katherine Heigl ended up hating the 'sexist' movie, Knocked Up.

Speaking up about the rom-com that first hit our screens in the 2000s, Rogen let loose on exactly how he felt about his former co-star and the negative comments she made about the movie in the years following its success.

The Pineapple Express star admitted that he felt 'insulted' by Heigl's claims over the iconic Judd Apatow flick.

It was pretty much a star-studded cast, comprised of some of the most popular comedian actors at the time, but Heigl, who mostly worked on romance movies like 27 Dresses and The Ugly Truth, seemed slightly out of place.

The plot followed Alison Scott (Heigl), a TV Host, and Ben (Rogen), a jobless bachelor, who ended up having a wild one-night stand.

Alison soon discovers that she is pregnant with Ben's child and the two have to come together to decide on their priorities, soon making a choice that could potentially change the fate of the rest of their lives.

With a humble budget of just $25 million, Knocked Up went on to take home a staggering $219.9 million at the box office.

However, much to the film's commercial success, it seems as if Heigl was far from impressed.

Back in 2008, Heigl dubbed the flick 'a little sexist', and even went as far as to criticize her own performance in the film during an interview with Vanity Fair.

Heigl told the news outlet: "[The movie] paints the women as shrews, as humorless and uptight, and it paints the men as lovable, goofy, fun-loving guys.

"It exaggerated the characters, and I had a hard time with it, on some days. I’m playing such a bitch; why is she being such a killjoy? Why is this how you’re portraying women?"

And it's clear that her comments did not go unnoticed, as Rogen and writer-director Judd Apatow soon weighed in on the matter.

Speaking on an episode of Howard Stern’s SiriusXM radio show, Rogen told all about Heigl's bold comments surrounding the film.

Rogen told Stern that he was hurt by the slam, revealing: "As we were making the movie, honestly, I was like, ‘I would make a dozen movies with her', before saying she 'improvised half her s**t."

"People seemed to like it," he continued, "We were funny together. I was having a really good time, and then when I heard afterward that she didn’t like it, that she seemed to not like the process, and she did not like the end product either, I think when that happens — also your trust feels somewhat betrayed."

The Freaks and Geeks actor then joked his theory over why Heigl ended up hating the movie: "I'm insulted that just the implication of that a woman would f**k me is sexist."

The pair went on to explain why they were so shocked about Heigl's criticism, as on set 'she could not have been cooler', with Apatow commenting on how 'hysterical' and 'fantastic' she was.

"Yeah," Rogen laughed, "she was awesome!"

After reading Heigl's Vanity Fair interview, the director recalled his reactions and how he thought: "I don't know what to make of that."