Three Questions about Sexual Desire

I am wondering a few questions in regard to sexual desire and Buddhism. 1. Is fantasizing a form of craving and thus worse than sexual activity without it? What are the mental processes involved? 2. How much does a single orgasm, for example, affect the spiritual path? Basically what is the magnitude of the effect? 3. Is it possible that some individuals cannot eradicate lust and should merely diminish it?