2. What about side characters? Is there always a certain amount of characters of a certain gender or do you also make this random?
3. Finally my question: How would one go about keeping gender anonymous? I don't want to tell the exact gender of the main character.
2. With minor characters this is most certainly random; I didn't in particular try to have only male or female characters as extras in say my chaptered fic, but then it really didn't matter much with them with what gender they had.
3. Just being careful with your description, I suppose. It's possibly more suited to other points of view, e.g. 1st or even 2nd (where the gender is the reader), but basically avoid telling the reader details that may suggest one gender over the other (e.g. 'wearing a dress' which'd be usually suggesting female), and the usual he/she/her/his/etc descriptors. Try not to overdo relying on 'it' instead though as that can be too repetitive.