My reproductive anatomy is neither inherently sexist, nor a social construct. And, in fact, sex (in the "which sex are you" sense) is not a social construct, though it is not quite as black-and-white as some people seem to think it is (for example, intersex people exist, and it appears to be the case that at least some trans people have identifiably cross-sex brains--ie on a brain scan, at least some trans men have male-looking brains, and at least some trans women have female-looking brains)
Gender, however, is (at least mostly) a social construct. There is nothing inherently feminine about, for example, skirts, or the color pink. There is nothing inherently masculine about tuxedos, or the color blue.
Some of the things we associate with women vs men are at least *somewhat* based on the average capabilities and/or characteristics of women vs men, but few if any are any kind of absolute. For example, the average man is physically stronger than the average woman, but there are women out there who can dead-lift more than the average man, and the difference between men's and women's average strength isn't *that* huge. So while more men than women are suited to be, say, firefighters, that doesn't mean all men can and/or no women can meet the criteria.