Always thinking of sex

Why am I always thinking of sex? Even when I'm not horny, or I could even have just finished having sex or masturbating and be very very not horny, but I'll still be thinking about sex. So, no matter if I want to have sex or not, in still thinking about it, or thinking about sexual things. And I tend to get horny very quickly after I have an orgasm again. Why? Is this normal?