Why do people like to have sex? I have always wondered this.

I have heard two different stories; one, that using a vibrator can do no harm, and two, that using a vibrator can do worlds of harm! I have heard that using one will desensitize the area and make sex with a real person totally unenjoyable and dissatisfying. Which theory is true?

Neither my boyfriend or I have engaged in sexual intercourse, with each other of someone else, but we do fool around quite a bit. We both perform oral sex on each other, but he says he doesn't like performing on me once I get wet. He tries his hardest to please me with his fingers but I think a full oral sexual experience would feel so much better. Why does he dislike this and how can we improve the experience for him?