I read somewhere that the reason they are male on Amonkhet is that they were created by Bolas rather than naturally (think Saruman and the Uruk Hai). It was a subtle story telling ploy to show the corruption and bastardisation of the plane under Bolas' rule. I always liked it. Subtley nefarious. One of the many reasons the flavour of Amonkhet is one of my personal favourites.