lol...It is normal for some cats. Be very glad you had him fixed early in his life. Some cats have more drive than others and he is pobably blessed that way. You are the "queen" of the colony and he is reacting as if you were indeed a cat. Most likely it will never become more overtly sexual than love-bites (gentle nips at fingers and ears) but among the cats I've had over the years and have now, a degree of sensuality happens. I've always considered it showed they were secure and comfortable. (Mine too are all nuetered)