The news site posted an article describing former rugby star, Cohen, as being "beefy", "portlier" and "out of shape" since his retirement from the sport.
The entire article appears to dig at Cohen by body-shaming him, with one section reading: "Perhaps the 36-year-old has indulged in one too many romantic meals with reported girlfriend Kristina Rihanoff."
The site has been criticised on Twitter, with one user saying: "Fat shaming is fine, as long as it's a guy. Good one. #everydaysexism"
Natasha Devon from the Self-Esteem Team tells HuffPost UK Lifestyle that the Mail Online's actions are simply not on - particularly as it promotes negative body image among young males.
"It's becoming increasingly acceptable to scrutinise and criticise male bodies, but, as we've seen with the crisis in confidence amongst women, the results of this are catastrophic.
"Men shouldn't feel that they have to be lean and buff to enjoy a beach holiday any more than women should feel the need to look like the Protein World poster."
Devon adds that her work is becoming increasingly focused on the mental wellbeing and self-esteem of teenage boys.
"Body image plays a significant role in this," she says, "with male eating disorders rising by almost 70% in the past decade."
Meanwhile body confidence blogger, Leyah Shanks adds that "this is an excellent display of the pressure men face to have the stereotypically 'perfect' male body".
"There can be a tendency to forget or overlook how much of a problem negative body image in men exists and how it impacts lives. This article is feeding the incorrect idea that one can only be happy with one's body when it is in said 'perfect' condition."
She continues: "The idea of only loving your body when it's in perfect shape is like only loving your kids when they're well behaved.
"Ben's body will be just as strong, capable of many great things and he will be the same person regardless of his current muscle tone. Athletes bodies are just like the rest of us - they vary in shape and size."