my neutured boy Marvin [he was neutered at the beginning of September] keeps mounting the little girl Molly [her spaying appointment is in a week or so, already made appointment]. she is in heat right now and he's suddenly started trying to mount her and bite the scruff of her neck. since it's been a few months since his neutering i know he can't impregnate her right? is this normal? should i be concerned!?