A stallion/jack is "proud cut" when the vet has not gotten all he was supposed to get during the gelding process. Sometimes, a gland is left which renders them, not completely gelded. It sounds like you are finding out about donkeys having "recreational sex"! Geldings often will do this and sometimes, the jennies almost force them to do so. At our place we have no intact jacks but when the jennies are in season they will respond to our mini stallions by opening their mouths, sticking out their tongues and/or mounting each other. We do have a gelded jack who also mounts our mini mares when they are in season. In all, I hear from the "donkey experts" that this is all "normal" donkey behavior!
His attitudes/actions might be caused by a recent gelding, also. I hope the vet had you keep him away from his mother for a few months after the gelding because they can still be fertile for a while.
I hope this helps calm you...
StarWish/Colleen