It is contradictory to want sex when one does not want children, even as it is contradictory to want to eat when one does not want to be fed - only for the "pleasure" of the taste... What I said about sex goes for food, too: it tastes good because it is good for you (in theory: for instance, sugar tastes sweet because it is pure energy. However, your tongue cannot foresee whether you're actually planning on using it instantly).