No healthy marriage relationship can compete with sexual fantasies. In fact, the addicted person prefers his fantasies to a relationship. A relationship requires two persons, requires effort and understanding, and involves disappointment. Since a love relationship does not exist, the kindness and fulfillment experienced in a love relationship do not exist. Sexual fantasies attack and destroy relationship skills by making the person increasingly selfish, self focused, and self centered as he increasingly is ruled by his feelings and desires.
Natural curiosity is a door.
The boy is going through puberty. His body is maturing. He deals with feelings and desires he never experience before. He is curious, and that curiosity is fueled by our culture's use of sex appeal. The inability to relate to other people is a door.