it certainly can be a stressor in a relationship both to the jealous party and their partner
Oh definitely! A mismatch in expectations of what is proper in a relationship causes a lot of problems. Jealousy in that case is often a culmination of unfairness, bitterness, betrayal, degradation, and possibly a sense of unrequited love. So Jealousy in that instance is just a suitcase term for a whole gamut of emotions related to a fundamental difference in expectations.
If the person just happened to be weak and gave in to some urge, then that complicates things even further.
One of the most common scenarios for jealousy in a relationship is one partner talking to members of the other sex. This can be seen as a trust issue, but there is a lot more to it. It is easy to run to someone else when things aren't going the best, and I think everyone is aware of that. So the perceived strength of someone's character plays a big part in that.
Sexual tension does exist, it is natural to have romantic thoughts of other people, it is just how we work. The thoughts we have no control over, but our actions we do. If one partner doesn't believe the other has sufficient willpower to resist an urge as strong as a helping hand (and perhaps a bit more) in a time of need, then you might call that jealousy.
But moreover, there is probably something at fault much deeper in the above example. If neither party is willing to help the other grow or if it is one sided, if they don't pitch in to help each other to advance their own image of themselves, or if one wishes to degrade the other, then there is nothing but fault destined in that relationship's future. To be strong, to have willpower, and to persevere, one must first have a strong belief in their own ability, not pride, not ego, not hubris, just a healthy image. After that all else follows.
The line between a healthy self-image and an unhealthy inflated ego is very fine. During conflict, the urge to rationalize and justify all kinds of actions is strong; the ability to resist is something only patience, dedication, competence, and experience can build.