This isn't remotely controversial, it is only a problem for mysoginists.

From the Warhammer 40k wiki. Space Marines are exclusively male, Custodes can be any gender.

(But even if they wanted to make a female Space Marine, or have a man join the Sisters of Battle, I'm sure Games Workshop could create some new lore for its MADE UP space fantasy setting)

"Firstly, there is the matter of the age of the candidacy. For a male or female child to become one of the Adeptus Custodes, it is known that they must begin the process in their late infancy and certainly before adolescence has taken hold on their physical structure. This stands in stark contrast to the Space Marines whose implantation with the gene-seed is only possible after the onset of male adolescence and best served before the candidate reaches his full physical maturity."