Of course you will ask how it is possible that somebody can be hurt by something that doesn't make him worse off, and in fact makes others better off. I think that the official answer has to do with misguided vanity. The jealous partner assumes that his personal characteristics, at least as far as they concern the interaction with his partner, are so much superior to those of the rest of humanity, that his partner can not have any need for others. When that theory is disproved in practice, his ego is hurt.
If this would really be the cause, the desire of ones other half to somebody else should be equally bad as an act of adultery. A partner that, due to a strong sense of morality, can avoid giving in to temptation, constitutes just as well a counterexample to ones superiority, as partner that falls for the attraction of the flesh, and is moreover a hypocrite. Under a strict application of this theory it is therefore not at all praiseworthy to resist the temptation to adultery, unless also hypocrisy is also seen as a merit.
Somebody who suffers from jealousy could argue that jealousy is an irrational and deplorable emotion, but one with which nature has saddled us, and with which we have to live. The rule of conduct to restrict oneself to one partner is then a rational way to cope with this emotion.
Considering the many relations that are devastated by jealousy, and the manifest presence in many partners of longing to thirds, such a rationalisation can be taken serious only when it is accompanied with the recognition that jealousy is indeed a deplorable phenomenon. However, once somebody recognises this, I expect him to lose his tendency to jealousy automatically. For I do not believe that humanity is saddled with inevitable social instincts that only make live miserable.
Jealousy is a cultural phenomenon, talked into people by their fellow human beings, just like the embarrassment some people feel for being naked in public. I therefore urge jealous humans to critically examen this emotion, if possible to suppress it, and to consider canceling the prohibition on multiple sexual partners, or at least not to apply it on ones partners.