Q:

Who are you romantically jealous of? [no joke]

Who are you romantically jealous of?
Say A (f) and B (m) are a couple, but B is way too friendly towards C (f) which makes A jealous. But who is A jealous of? B or C?
Or C (f) has a crush on B (m), but A (f) and B are a couple which makes C jealous. But who is C jealous of? A or B?
Basically what I want to know is the target of "jealous of" when it comes to a romantic relationship. Cases like "I'm a dude and I'm jealous of him because he has a girlfriend" are pretty clear as to what the target is since you can't really be jealous of the girlfriend as you're a straight dude.

A:

The word jealous, like many common words, is polysemous: it has multiple glosses enumerated in a good dictionary like the American Heritage:

Envious or resentful of the good fortune or achievements of another: I felt jealous when my coworker got a promotion.
Fearful or wary of losing one’s position or situation to someone else, especially in a sexual relationship: Her new boyfriend was jealous of her male friends.

. . .

Vigilant in guarding something: We are jealous of our good name.

. . .

Your last scenario corresponds to #1 here in being a matter of simple Envy (the only one of the Seven Deadly Sins that is no fun at all). But when, in your first scenario, you say that A is jealous, A may be jealous of C in sense #2 (as illustrated in the American Heritage example) or of B in sense #4.
So in that first scenario, where A and B are a couple and B’s apparent over-friendliness towards C makes A anxious and upset, your question whether A be jealous of B or of C can only be answered yes. It’s ambiguous. Natural language is like that. Get used to it.
And speaking of getting used to it, note how describing the scenario becomes easier and more (ahem) straightforward without that anxious death-grip on hetero-normativity.

