Obviously, opening a relationship was a can of worms in itself but it seemed like Rachel and Jennifer had a lot of stuff to talk about yet refused to do so. Communication was always key in these kinds of situations and the two chose a band-aid "fix" to their relationship, not an actual solution.
Allowing one's partner to sleep around in an open relationship or a semi-open one had a lot of reasons from couple to couple or anything in excess and in between.
One could be that one or both of them needed to explore because they missed out on a lot of things, wanted to try something they couldn't with their partner, had straight-up curiosity, had this want to involved more people in their relationship sexually or emotionally, etc.