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Making the gem only work for Happosai limits future options. You know he’s never going to use it on the fiancees, for example, and that line means no one else could either.
Happosai will be using the Gender Gem on women! For the same reason he'll never fully feminize Ranma. It's a well established fact in the Ranmaverses that Happosai's a pantie raider because he draws vitality from the latent yin auras of his precious darlings. He has to steal, and has to keep on stealing, a lot of the scanties because of how low the quality of that gender energy is. But it makes sense that he would want to take the energy more directly. He can't as long as the yin is as firmly attached as it is to a normal woman.
Ranma has both energies maneuvering about in him/her. His yin (the energy that conflicts with his normal energy) is not as firmly attached, easier to draw on. That is why, for me, he's stopped hanging onto girls as he had the first time we met him, but does still try to latch onto Ranma.
Using the Gender Gem ring, he'll be able to set up the yin-yang instablity in more people, including the fiancees. (He is going to have to push them so that yang becomes their dominant energy, so their yin energy can be grabbed. - Oh, no! Not sweet, feminine Kasumi!)
But what Happosai forgot was that mixing yin to his own yang is exactly how he raises his own vitality! If anyone with a yin-yang vortex discovers how to make the unstable more stable (that is, drawing in rather than throwing off the conflicting energy), they will become more alive with chi ->ki(applied chi). They will have more vitality than Happosai! But they could become even more perverted.
As for a loophole, there are the Past Year Journals. Their magic is now neutral only to Happosai. And there is more than one. And with people with with unstable genders setting up new recent-past biographies who knows what could happen. And what would happen if two or more of the journals conflict in details?
I see. That’s a lot to process, but now I get where you’re going with this.