While In sure a lot of what's happening is motivated by insecurity, it's also completely valid that you simply do not want a poly or open relationship. Which is equally as valid as wanting a poly or open relationship.

I noticed you said you are extremely healthy and communicate well with your partner. Based on what you have stated It sounds as though that is not the case.

Furthermore, anytime a spouse wants to seek an open or poly relationship based on their current partner not satisfying their sexual needs, it's almost certain to fail and cause issues. You should work through your sexual issues together as a couple, and I'm honestly surprised that wasnt your spouses priority when faced with sexual issues. If your relationship was strong and you had excellent communication and prioritization of the relationship, this would be the natural progression. Like all things stepping into an open or poly relationship is suppose to add to a healthy person and relationship, not replace what is missing.

Trying to fix a problem by looking for solutions outside of the relationship is certain to cause issues.

I'm not surprised you are having the reactions you are having, your spouse was unsatisfied with your sexual relationship and decided to look elsewhere instead if working on a solution with you. Any healthy person would feel hurt and upset by that and likely experience insecurity, because that is literally a relationship that's security is waning.

If I was in your situation I would pursue couples counseling and have a serious talk about both your priorities. Open and poly relationships need to be healthy internally before they can be healthy externally, as is the nature of all things with being human.