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Mansplaining incoming:
You see, we are animals and it's on our genes to, in some point of our lives, procreate. The rules inscribed on our genetic code tells us men to search for a female with maternal instincts and a fertile body (child-bearing hips, feminine facial features and big breasts) And for women: a provider and phisically and mentally capable male.
In the course of life, both males and females develop feminine or masculine traits to ensure reproduction. And the reproduction, unconsciously or not, is a big deal, the individual forges in its mind an ideal of the desired self and the desired mate. From this desire comes the graphic representation of idealized bodies.
To extend on this: I, as a male, find pleasant to look at a woman with the traits I mentioned before. And seeing a fit male body does not affect me because I know that if I put enough work I could achieve that.
I suppose that the sight of a fit male body is desirable for a woman too, or at least, not disgusting. But the current trend of "body acceptance" and the mass-media message of "everyone is special" perverts the mind of the young females telling that it's bad to put work into better themselves and cultivating feminine qualities. Then, the ideal of an apt body instead of serving as motivation translates into jealousy and bitterness (This negative feelings could be amplified by the innate sense of competition of humans)
Now, lets put for example, a pretty girl in a bikini on an advertising. The message of the image goes from "you could look this cute wearing our clothes" turns into "you will never look this cute you fat b*tch"
In conclussion, the so called "objectification" of the female body is almost always done by women themselves. And the "sexist" representation of human bodies (even for commercial purposes) is mostly a representation of an ideal and the appealing to the primal urges of human beings.

PD: Hope my grammar is okay, I'm not good at it.