Why is it okay to see women in their underwear/bra on TV, in magazines, on Instagram & Facebook, showing their tone…

Why is it okay to see women in their underwear/bra on TV, in magazines, on Instagram & Facebook, showing their tone bodies, their fat asses, their big breast BUT when a pregnant women makes the decision to post a picture of them breast feeding their baby or post a picture of their growing bellies in underwear/bra or maybe even naked but of course covering their breasts, it becomes a problem. Why is it okay for some women to do it but when it comes to showing what our bodies are made to actually do it turns into us looking for attention, into pregnant sluts, into "we need to be modest" into "we shouldn't be mother's."