I think this is an important place to emphasize the distinction between jealousy and envy.
In a committed relationship, you have more to lose, so there's more opportunity to feel jealousy. And because ideally, a person wants their partners to be happy and fulfilled, so they may be more likely to feel compersion rather than envy.
In casual relationships, you're more likely to see your lovers out and about with their other lovers, especially if you all have overlapping social circles, and this can create an opportunity for envy. I suppose there's also an opportunity for jealousy, in that you may be in a casual relationship with someone and you see them growing a stronger relationship with one of their partners, which may "push" you out a little bit, leaving less time for you.
I guess "opportunity" isn't exactly the right term, unless you choose to take ownership for those emotions and conquer them, thus making it an opportunity for growth.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker