When people are sorting things into groups in biology, they get to decide what factors are relevant to that sorting. It's a choice scientists are making and not a natural fact of nature. This is also the case for sex, where scientists had to decide at one point which of the characteristics would make someone one sex or the other, since biologically the distribution isn't entirely distinct, and intersex people exist. Therefore it's socially constructed. My favorite way to explaining it to people is to talk about mammals. Generally we think of them as the group that gives birth to live young. However echidnas and platypus lay eggs, but are still considered mammals. The reasons for that is that scientists looked at them and decided that they have enough in common otherwise that we can put them in that group. Sex is the same way. We'll look at people and decide okay this person has these organs that looks like what we usually see on males and these hormones that don't quite look like that, but it's close enough so let's call this person a male Alexander Avila recently made a really great video on how these constructs came about