A U.K. publisher is apologizing for claiming in a book for boys that one of the reasons women develop breasts “is to make the girl look grown-up and attractive."

First released by Usborne Publishing in 2013, Alex Frith's Growing Up for Boys is advertised as a “frank and friendly" guide for young men on what to expect during puberty. But when it comes to explaining female puberty to the lads, the book falls short.

A section titled "What are breasts for?" states that "Girls have breasts for two reasons: One is to make milk for babies. The other is to make the girl look grown-up and attractive. Virtually all breasts, no matter what size of shape they end up when a girl finishes puberty, can do both things."

Simon Ragoonanan, who writes about raising kids without constraining gender norms on Man vs. Pink, posted photos of the passage.

Other Twitter users were quick to express outrage.

"Not just horribly sexist and objectifying," one wrote, "but the implication is that as soon as a girl starts developing, she's 'grown up.'"

Another user wondered, "Does the girl's version talk about how boys develop body parts purposely for looking grown up and attractive?"

"Was this written in 1950?" asked another.

A few people chimed in to argue that the book's stance was valid since, as one man put it, "most men find breasts attractive." Of course in the real world not all women have breasts, and not all people with breasts are women. And a lot women don't judge their attractiveness by their breasts.

Ragoonanan even offered a sarcastic followup: "Girls have legs for two reasons: for walking and looking grown-up and attractive."

Following the Twitter fallout, Growing Up received a slew of one-star reviews on Amazon.

“Surely we can do better than this in terms of what we teach our sons?” a reviewer asked.

"I'll be looking elsewhere to find a book to explain to my son what's happening to him as he grows up," another wrote, "One which doesn't treat women and girls as nothing more than their body parts."

Following the backlash, Usborne Publishing apologized for the offensive chapter and promised to revise the book for its next edition.