i will explain to you why this is. in my opinion it is because a pregnant woman, seeing her with her belly protruding that is very conspicuous of her being fertile and sexualizes the woman more. like fat on the hips , breasts and belly makes it more obvious overtly that a woman is sexualized object. but on the magazines and shows you are talking about there they show slim girls who look nothing like the fertile sexualized image of the woman yet instead sort of resemble the pre-pubescent teenager. if you still look like a gawky teenager or a prepubescnt girl you resemble more to a young boy so this image is far or even opposite of the fertile sexualized image of the woman. that is why people are not bothered by it more.
so whatever seems to people in the public an overtly sexualized image of a woman makes them feel weird or uncomfortable. well at least in this case the pakistani audience.