It depends entirely on the person and the relationships, I think. I don't really get jealous, or even envious much, in my current relationships. Some of that is to do with my personal mental make-up, and some of it is due to the fact that as I was entering an existing relationship I was fully aware that time, attention etc would be at a premium and would take some negotiating. I came into the relationship having heard about my boyfriend's wife on a regular basis, so was fairly prepared for having her in my life and having to share him.
It also helps that we're all very aware of someone being left out, so we go to great lengths to remove causes for jealousy before the feelings arise.