More Thoughts on Jealousy

It's an interesting proposition that the emotion of jealousy/possessiveness -- in this narrow context of marriage/romantic love -- is not necessarily sinful or ethically wrong (unless it is taken to extremes! The same can be said of most emotions...) Andrew and I were working on parsing out the possessiveness aspect of jealousy yesterday, in an attempt to understand why it appears to be universally the case that the presence of a third person in the breakup of a marriage would make the whole thing much more bitter, than if the two people just fell out of love. Should there be any difference in the dumpee's emotional reaction to a break up due to infidelity, versus a break up due to the dumper's just getting tired of the relationship? What do we call that element that injects the extra bitterness in the first scenario, and, whatever it is, is it morally wrong to feel that way?