Oh, I firmly believe that jealousy is natural, impossible to counteract, and oftentimes beneficial...
...however, the rule is NOT about preventing jealousy, but about preventing the un-Gorean effects of jealousy when they are applied to an online situation.
Were slaves jealous of other slaves? Oh, goodness, yes -- and such jealousies often made them better slaves when they competed to catch a desireable man's eye. However, they did not dare dictate which slave a man should to take to his furs. To do so would be to encourage a severe beating. If she lost the "competition," that was the end of it. In at least one instance in the books, a slave was sold for so much as daring to suggest that a Master should have chosen her to adorn his leash, rather than the slave to which the leash was clipped.
Were slaves jealous of free women? Oh, goodness, yes -- and they knew they'd better tiptoe around such powerful, wrathful, vain, and prudish women, lest their lives be made a living hell. The books tell us that slaves almost always hid their sexual needs when in the presence of free women, even if this meant failing to catch the eye of a desireable man. And you'd better believe that a slave had to know a Master extremely well before she could dare to joke about the petty mood swings and prudish ways of free women.
Were free women jealous of other free women and slaves? Again, absolutely. Free women sometimes took great pleasure in humiliating the women in their social circle, especially if those other women were extremely beautiful and highly sought for Companionship. It goes without saying that a free-born woman was incredibly cruel to the female slaves of her household, especially if those female slaves were couch-warmers for her Companion. And yet the free woman of Gor usually knew better than to shrewishly forbid their Companions to make a journey to the local paga tavern or bath house. Several times in the books, free women who attempted to interfere in a man's sexual festivities were beaten.
In all of these instances, the jealous women knew better than to dictate to a man where he would take his pleasure. Regardless of her feelings, the choice was his, which was his right as a man on Gor.
And now we come to online, with its whispers of "don't serve him too sexually, he's already taken," and "I think you need to back away from my man." You see, most of the jealousies are ALREADY being taken to pm and ruining role-play. If you haven't seen it IN role-play, that's a good thing -- but that doesn't mean that IMs and PMs haven't been flying left and right.
Yes, jealousy is a human emotion, and Goreans are humans... but the RESULT, the BY-PRODUCT of this ONLINE jealousy is the very UN-Gorean phenomenon of seeking to a control a man's sexuality -- and that is what prompted the rule.