I see the reason for the rule. Sexual activity leads to mental dullness and laziness. It's like an intoxicant, but which is the most subtle hindrance to mindfulness. Afterwards, you might take a nap and feel a bit better. But does anyone notice the effects after you take a nap and wake up? If you have undone the chains which bind you to kama-dhatu, it feels as though those chains are restored. So, you engage in sex, and once you have had a taste, then you are on the road to engage in other forms of sensual desire.
What do the Mahayana texts say about bodhisattvas and sexual restraint? I have noticed: If you practice self-restraint, from abstaining from sexual activity complete and (for men) from mental restraint during the moments leading up to orgasm, sex is much more enjoyable. With the "mental restraint," during orgasm, I'd use the admittedly crude analogy of a sling shot. This is useful for couples, I think, because I think that sometimes couples lose sexual interest in eachother merely because they have indulged in sex with eachother too much, to the point that it becomes boring. If they only engaged in the activity spontaneously, it would be great during the whole lifetime of their relationship.
Anyway, with other intoxicants, it's simply a matter of seeing the danger and cutting off the object: the friends who use the drug and\or the mental state of craving for it, through no longer self-identifying, "I am a cigarette smoker," for example. But with sex, how is this possible? It feels biological and there are beautiful women everywhere, and now that I rightfully practice Zen, it is easier for me to charm them (or at least attempt and fail many times, and statistical probability says eventually I will succeed!
Is it possible to engage in sexual activity "mindfully"? It doesn't seem so. At least, not in the sense of avoiding attachment. One is merely mindful of the craving involved. It's not like with food, where you can eat something delicious and not develop craving, because you are merely dispassionately taking care of a biological need. Could sex be like this: a biological need?
I am not a puritan, by the way; don't misunderstand me. I'm not opposed to sex -- for laypeople, they can do whatever they want. But it seems like sexual activity unnecessarily clouds the mind. I'm not a monk, but I would like to understand that.