Pagan priest said:
Okay, why? Not trying to be sarcastic or anything, but why do you think that promiscuity is in any way a "not good" behaviour? You mentioned natural law, but how does that apply here? I am aware that numersous real world religions consider casual sex un-good, but how does that translate into D&D, when (as I mentioned) so much of that really seems to translate to the lawful/chaos axis?
OK, I'll take a stab at it. I'll try to frame it in a way such that it can be abstracted from specifically Hebraic affiliations.
Anything attains its excellence insofar as it imitates the eternal (and if it is already immortal, then insofar as it imitates the most purely eternal). This is the basis for the cycle of life, evolution and weal. Each different kind of thing is relatively more or less capable of such imitation (a dog moreso than a daisy, etc.). The basic proper orientation of life (according to the view I am representing to you) is to preserve and produce life. The basic mechanism for the production of life is sexual procreation.
A person does not have to be procreating all the time (time must be spent preserving life, and for human beings there are also other things to be done, as I will explain in a moment). But when one does engage in sexual activity, it would be contrary to our natural excellence to deliberately frustrate the act's capacity for fecundity. The end of the act is procreation. That it can be enjoyed is secondary and a result of the passions (but enjoying it is not bad, when it is done under the right circumstances). Of course with human beings, who are rational, there's a further and in fact more perfect way of imitating the eternal: contemplation of the truth. There is even a 'procreative' aspect to this when the truth is conveyed in conversation. And the highest form of life is the contemplation of the highest truths. However, this does not mean that the other activities, including sexual procreation, are to be discarded in the pursuit of the most perfect life. On the contrary, these activities are related to the superior activity of contemplation as subordinates, just like horse grooming, bridle-making and cavalry tactics are subordinates of generalship (though each and every subordinate activity is not necessary to produce the superior; however, when a subordinate activity is undertaken it must be undertaken in a rational way or else it will undermine the imitation of the purely eternal).
See especially the speech of Socrates (really, of Diotima) in Plato's Symposium, and Aristotle's books Physics (especially Book Two), On the Soul and Nicomachean Ethics. The Stoics are also helpful in this regard.
This is not an issue of Law and Chaos in my view because those (in the 9-part system) are methodologies. A Good character who is strict, disciplined and a disciplinarian is Lawful; a Good character who is easygoing, undisciplined and "mushy" is Chaotic. Lawful tries to instill goodness with a ruler to the knuckles and a rigorous regimen; Chaotic tries to instill the same morality with group hugs and homemade cookies. Lawful tries to bring about change by "working within the system"; Chaotic tries to bring about change by "bucking the system".
I should note that as primarily a Classic and OD&D guy, I tend toward the 3-part alignment system, which has no alignments for "Good" or "Evil" (nothing codified, anyway).
Now to consider the specific issues of chastity and promiscuity: I consider chastity good because it is fully in accord with the rational (procreative) nature of the sexual act. Nothing says that you have to procreate, though you are bound to imitate the eternal in some way, so if you cannot or will not do so sexually then you ought to channel that energy into the rational form of imitation, i.e. contemplation and rational activity; this shows how abstinence can actually fuel your pursuit of the pure as a subordinate act. Now, what I would say about sterile promiscuity is surely obvious. But what about fecund promiscuity? I consider this contrary to reason because it erodes the family life, confuses children, and presents a barrier to the complete gift of spouse to spouse... that is, a spouse is required to give himself or herself over to his or her spouse in a unique and central way which is prevented if sexual activity were being practiced outside of the marital dyad.