Envy consists in wanting what someone else has. Though potentially unpleasant, it’s understandable enough, as it follows naturally from a positive appreciation of the object’s value. Jealousy, by contrast, is the purely negative emotion which involves wishing that the other lacked the object of value. (I mean for these definitions to be stipulative.) Such an attitude seems quite thoroughly unreasonable, at least if the object in question is shareable. For rivalrous objects, one might want the other to lose the object solely as a means to one’s own gaining of it. Such “instrumental jealousy” lies in the service of envy, and so may inherit the latter’s reasons. But to begrudge another’s benefit, for its own sake and not for costs to oneself, seems blackly indefensible.
What, then, are we to make of romantic jealousy, i.e. the possessive desire for our partner’s exclusive attention? Is it simply unreasonable? Is the value of one’s relationship necessarily diminished in virtue of its non-exclusive character? Surely not: the value of a relationship is grounded in its deep or intrinsic character, not merely incidental or comparative aspects. (One occasionally hears homophobic rhetoric implying that one’s marriage would somehow be “devalued” if gay people were allowed to marry too. But this is absurd. Truly valuable relationships do not rest their value on such fragile foundations. Other discussion critical of zero-sum “comparative” values can be found here and here.) What matters is your relationship, not any other one – even if the other involves your partner.
(Granted, it may just be intrinsically unpleasant to imagine your partner having sex with someone else. But while we can give an evolutionary explanation of why such ‘pure jealousy’ might evolve, that doesn’t suffice to justify it. One can also give scientific explanations of violent anger, but we should resist such pernicious emotions all the same.)
So any reasonable grounds for objection here will need to be grounded in concern that your partner’s activities will negatively impact upon your relationship. There are two obvious ways this could happen: sharing might mean less of them (their time, attention, or affection) left for you; or worse, they might leave you completely.
It isn’t clear whether fear of breaking up should rationally make one more or less jealous. Presumably the worry is that non-exclusivity would give your partner more opportunities to meet and get attached to someone new. But on the other hand, it also reduces the incentive to break up if they do find someone else. In particular: if your partner still likes you, albeit less than they like the other, then non-exclusivity might in fact save your relationship, whereas jealous ultimatums would force them to break up with you. (The situation is complicated if the other becomes jealous of your partner’s continued attachment to you. The next section will address whether their jealousy could be reasonable.)
What of the concerns about ‘sharing’? They sound to me unreasonably possessive. Your partner’s time and attention may be taken up by any other hobbies or interests that they have, but those are not legitimate grounds for complaint (unless you are being thoroughly neglected). It isn’t clear why an interest in another person should be treated any differently, at least on those grounds.
I take it the real worry here concerns affection. Some may implicitly believe that individuals have a fixed emotional capacity, so that the more they care for someone else, the less they care for you. But when made explicit like this, such a view does not sound very plausible. Consider parental love. Surely nobody would claim that children from large families are loved less by their parents than is an ‘only child’. But why should romantic love (or its precursors) be any different?
Perhaps we want to be “special”; but it isn’t clear why exclusivity should create added value here. As previously noted, any non-deluded evaluation of one's partner needs to be consistent with the recognition that they’re not uniquely special, i.e. special in a way that everyone else fails to be. Our ‘specialness’ needs to be consistent with other people being special too. So, as noted above, we should look to intrinsic rather than comparative values. We have value for who we are, considered in ourselves, rather than considered in comparison to other people.
That’s all well and good from an objective point of view, but we still want to have a special significance for our significant others, even if it is recognized as a merely “subjective” or agent-relative importance. But we can grant this without requiring total uniqueness. Again, the parental analogy is illustrative: a child wouldn’t want her parents to treat her no differently from all the other kids in the world. She should have a special place in her parents’ world. But she needn’t be the only person in this place; she can share it with her few siblings, without diminishing its value in any way at all. So again: why is the romantic case any different? If someone has two significant others, must they be the less significant for this?
Perhaps I’m missing something obvious, since my armchair certainly doesn’t offer the most comprehensive view of the world. But at least in light of the issues discussed so far, it seems to me that jealousy largely is irrational, so that more reasonable creatures would not get so possessive or hung up on issues of romantic exclusivity. Whether we’re capable of being more reasonable creatures is, of course, another question entirely.