I find the whole discussion either hopelessly arcane or else blindingly obvious. Being attracted to someone isn't a reason for having sex, any more than saying "the food smelled good" is a "reason" for eating. Human sexual arousal is as much an animalisitc response to inner and outer conditions as it is a mental decision. If you and I are talking and you begin flirting with me, that isn't about a "reason" for having sex.

If there are "reasons," it's precisely those times when we're either not aroused or accutely aware that we should not have sex. A woman who gives in to her boyfriend because she's afraid he's going to leave her, or a married man who allows a seductive woman to persuade him to have sex are thinking about their motivation. If you and I are unencumbered by outside forces, then our having sex is simply the natural progression of attraction and not a reason at all.

Tom Paine
author of "Polyamorously Perverse"
http://perverselypoly.blogspot.com/