After being entirely monogamously married for 13 years, my husband has recently had a self-described philosophical "awakening", in which he has decided he doesn't and probably hasn't ever really believed in monogamy, and he would like us to open our marriage.

He claims he would feel nothing but happiness and compersion for me, should I want to start dating and exploring connections with other people.

I can't say I can relate to this at all. I want him to be happy, and of course the thought of him being happy makes me happy as well in most contexts - so why not this one?

I am an inherently introverted person, and would not feel like I were "missing out" on time with him at all should he want to go out in the evenings on a regular basis to do literally any other hobby. But something about the thought of him dating, and having deep emotional connections to the same level as ours with other people just makes me feel like I'm being stabbed through the heart.

Where do you think this type of pain comes from?
Is it ingrained in us biologically/instinctively, or is it mainly culturally learned? It seems like many ENM/poly people still often feel pain when their partners are connecting deeply with others. Can you "unlearn" it? Has anyone actually been successful in doing so?