It kinda bothered me that the game decides the slaves servants gender purely on the genitals with no way to overwrite it... so I just added one.
https://mega.nz/file/gJdQQSpY#SB8GVj6WK689wCD4oUzxtxyMh4H73agKqhvW4BHDxfg
This mod just adds a talk option to your servants to set their gender regardless of their bodily equipment. (choosing "decide for yourself" will set it back to the vanilla system)
Keep in mind that it might result in unintended sideeffect (like the chance to cheese sidequests by changing the gender of a servant), through I cant think of any other problems right now.
Update: Its now possible to marry a NPC at the slave traders. Doesnt do much atm, but maybe someone wants it for roleplay purposes.
Update2: You can now also addopt slaves as your children or siblings. Again, it has no impact aside from roleplay.