I'm noticing the running theme of "rightfully yours". Could you define what that means in this context?
If something is "rightfully yours" then it's something that you either "own" or have been "given".
Bex and I have "given" each other our lives, our love, our sexuality, our sensuality, our friendship, etc. wrapped up in a relationship and defined in very specific ways. Either of us can justifiably experience "jealousy" whenever one or the other gives so someone outside our relationship anything that is defined as "ours" within the relationship.
So, for instance, penetration is something that we have defined as requiring a committed relationship. If Ben goes out and starts up a relationship with another woman and gives her penetration, Bex can be legitimately jealous--because we have defined that as something that happens only within a committed relationship that we have both agreed to.
If Ben desires to be with another woman who is married and their marriage relationship does not allow for her to freely give her sexuality to another, then he would be "envious" of her (no "rights" to her).
Envy could also be defined as seeing his buddy's awesome custom Harley and wanting *that specific Harley*. That specific Harley is not Ben's and he has no rights to it whatsoever. So, that would be envy.
If Ben took his buddy's Harley, and his buddy wanted it back, his buddy would "jealous" that Ben had his Harley (presuming that Ben took it unlawfully). If Ben paid his buddy for it and his buddy wanted it back, then his buddy would be envious--because he no longer has rights to it.
In our faith, God has the right to be jealous of any worship of any other thing than Him--because all worship is rightfully His, since he created us to worship Him. He does not demand our entire love and affection and freely gives us the right to love one another (in fact, He commands it). But our worship is His and His alone. Thus, "Our God is a jealous God" implies no sin on His part--because our worship is his right and when given to another, He is rightfully jealous of whatever has stolen what is His right.
Does that help you understand our perspective?
We think it applies quite well to poly relationships because how you define your relationship helps to define whether you are envious (having no right to lay claim to something) or jealous (having every right to lay claim to something).
If you say your spouse is "free to love another"--then you feel those tugs of stress over it, then you're feeling envy, not jealousy. If you have defined it as a particular set of circumstances (only on alternate weekends, and only oral pleasure) and they step beyond those rules (weekdays and/or penetration), then you have the right to be jealous of something that was yours and taken from you.