The boy clothing features a Superman logo and the line "Future Man of Steel," while the girl item has the same logo but the line: "I Only Date Heroes."
"It seems kind of ridiculous to talk about who an infant girl is going to date," said Aimee Morrison, an associate professor of English at the University of Waterloo.
"Even for tiny babies, we seem to think of girls as gaining power and worth from whom they're romantically linked to and boys get to become agents of action in their own right."
The photograph was taken on the weekend at a local target store by Morrison's friend, Christine Logel, an associate social development professor at the university.
Logel was out shopping for toys for her two daughters aged 3 and 8 when she saw the sleepers made for three-month-olds.
"My heart really sank," said Logel, a social psychologist by training. "I'm tired of seeing these messages everywhere."
The assistant store manager thought they were "cute," she said.
Logel initially put the picture of the PJs on Facebook but gained little traction. The women then decided to share the image on Morrison's Twitter account, sparking hundreds of retweets and comments.
The response was overwhelmingly opposed to gender stereotyping, with many responders passing along their own examples.
One person sent her a photograph of a similar set, with one shirt saying, "Training to be Batman," and the other saying, "Training to be Batman's Wife." Another said she found the same Target outfits in New Brunswick.
Responses came from as far as Spain.
The girl shirt in Spanish said, "Beautiful like Mommy," while the boy one said, "Smart like Daddy."
The point, said Morrison, is that the stereotyping found in the Target clothing was not an isolated occurrence.
"They were really very easily and very quickly able to send me picture after picture that demonstrated this deeply held but not explicitly discussed bias to thinking about girls and women in terms of their sexual currency and men in terms of their capacity to conquest," she said.
Not everyone saw a problem with the clothing.
"There are more important problems in the world like world hunger," one tweeted.
"You obviously don't have anything important to do," another posted, while yet another urged her to "grow some balls."
Target refused to discuss the issue but released a statement saying it "strives to treat all our guests with respect, and it is never our intent to offend anyone.
"We appreciate the feedback we've received and will continue to listen to our guests to ensure we offer merchandise that appeals to, and reflects, our diverse guest population," the company said.
Logel was not impressed with the company's response, saying she "really thought better" of Target.
However, she was gratified by the social media response.
"The most potentially disturbing part about pyjamas like that and similar outfits: It does affect how adults treat babies," Logel said.
"(But) I'm really thrilled to see how many people want little babies to have a chance to be whatever they can be as they grow up."