that would be worrisome
as what he is essentially saying is he might not be poly compatible as he may not be able to handle his partner or himself having more than one friendship that includes sexual interactions.
That kind of talk without including words to convey that the breakup is really affecting him because he caught himself experiencing completely unnecessary jealousy, which he knows can ruin relationships unless it is rationally addressed, and so he just needs to take a deep breathe
There is absolutely nothing wrong with not being able to be in poly relationships, in full disclosure many days the attitudes that seem specific to "poly" people piss me off much more than "mono" people as often the level of intrusion into private life seems much more acceptable to those who identify being of the poly persuasion
so I have respect for monos the same as I do for poly people, but a person who doesn't have issues with non-monogamy only when they are the ones that have more than one person they engage is sex acts with I begin to have trouble holding onto respect for them. As it is a warning sign that they are not mature enough to act like an adult or else they do not know themselves well enough to make their own decisions without their choices clearly showing a lack of respect for you.
It becomes increasingly hard to respect yourself when the people you spend time with do not respect you. In fact unless you are a prisoner, it is impossible to not loss respect for yourself when you are around those who do not respect you as choosing to continue relationships wherein you are not respected is the same thing as telling yourself directly that you are not a person who deserves to be respected.
Eventually you will believe those lies, and if you believe them for long enough period of time they will unfortunately become a true reality of your life
all emotions and feelings are valid, but paying attention and being aware is necessary so that you do not inadvertently let your emotions present themselves in manipulative ways because if you do it will only cause misery and unnecessary pain and suffering which is only enjoyed by very specific sadists and only when coupled with just the right masochist.