I feel the same feelings.
I have exactly zero jealousy for my wife, since I am absolutely secure in our relationship; I know that we feel the same way about poly in general and our relationship in particular: that our feelings for and sex with other people has no bearing on our relationship with each other. Our relationship isn't remotely in any danger when she has a date with someone else or goes to bed with them.
On the other hand, I have a big crush on a mono girl right now, and we do a lot of "platonic dating:" i.e., we spend all kinds of time together and share a certain intimacy without being in an official relationship beyond "friends." I know that some day she'll find the monogamous hunka burnin' love that she's looking for. When that day arrives, what she and I have now will be gone. And holy crap do I get jealous of her. Scary jealous. Because I know she is mono and our intimacy is ephemeral. It could end at any moment.
And that's jealousy, because it's fear of losing something I have. There's no one to be envious of. Although I'm sure I'll be quite envious when that dude comes along.
I was actually thinking about this recently. Mono people get all jealously bent out of shape over sex. But I can't be jealous of sex with my crush, since we have none. Poly people get jealous over time.