Men have their gonads outside of their bodies and they need to be below body temperature to function properly. When they are too hot, they send the signal of pain to brain. Hopefully this causes an action that reduces the temperature of the gonads and prevent damage. Some men probably have other physical characteristics that exacerbates this heat problem that makes them to want to have their legs spread more than other men. If seats are empty to one side, the spreading of legs should not be seen as some attempt of male dominance, if the man brings them together when someone else comes to sit next to them.
How the term "manspreading" is used, while so many other words are forbidden as offensive, puts on clear display the double standard that many feel is held by mainstream media.