Biologists know what sex is. The thing is that we are used to mentally thinking of living organisms as collections of their cells so we can analyze their inner workings. It's a sort of reductionism. But we understand that these parts don't function individually. They need to be seen in the context of the whole organism to truly understand them. We are more than the sum of our parts. That is, the information gained individually from each part is insufficient to explain the behavior of the whole organism. In order to understand the whole organism you need a further level of information that you can only obtain by looking AT the whole organism; you need the way the different parts naturally relate to each other. A uterus does not live on its own in the wild, foraging for food or randomly producing babies. It is always found in the body of an actual woman.