I think I assess maturity based more on behavior than internal reactions. I don’t think people can control their thoughts. I do not have wonderfully mature thoughts. To use a minor example that I’d rather not confess, my friend was in town visiting and told me that while he shipped my husband’s Christmas present directly to our house, he forgot to get mine and would have to ship it to me when he got home. I am a grown-ass adult, but my internal dialog was something like, ''Motherfucker! Why does Sr. Weasel get a present on Christmas and I don’t?" When it comes to presents, I’m about as emotionally mature as your average seven-year-old.
My *response * was something like, “We’re just really looking forward to seeing you.” That is a mature response.
To use a more extreme example, I was raised by a person with a violent temper, and for either genetic or environmental reasons, I seem to have inherited it. There are times where I just want to take a fucking baseball bat to all of our furniture. But I don’t. Because I learned early, thanks to some good modeling by my beloved Auntie, that is not a mature response.
So yeah, someone can be inclined toward jealousy, but it’s how they behave that makes or breaks it, IMO. And when your partner comes to you and says, "I feel absolutely awful about this but I’ve developed feelings for another person but the reality is I want you and I am so lonely without you…‘’ you can choose, regardless of feelings, to respond in any number of ways, ranging from “That fucker is never allowed in our house again’’ to ''Let’s have a threesome.” Somewhere along that spectrum are a number of responses that are more mature than others. I think my husband’s response was profoundly mature.
My view about what a mature relationship looks like is admittedly entirely subjective. It is influenced by experience on the extreme end of immaturity. My Mom’s treatment of me aside, her numerous romantic relationships were godawful and full of paranoid, irrational shitfits and weird jealousies that made everyday life an endless drama of suspicion and betrayal. It was so fucking idiotic and when you’re a kid you can’t exactly jump into the middle of an altercation and say, “Do you realize how fucking unreasonable you’re being? Is this really worth your relationship?” (My mother’s five marriages and her string of broken relationships are another reason that the idea of monogamy = mature is inherently laughable to me.)
So I was tasked, as a young adult becoming involved with a man at a very young age, to fashion a relationship that flies in the face of what passed in my household growing up. I had the advantage of experience and observation about what doesn’t work as well as an innate sense of goodwill. Unlike my mother, I do have a high degree of empathy. But here is the terrible truth about me: I’m not any less emotionally volatile than the woman who raised me. I’m just damned good at behaving otherwise. I want to yell, I don’t yell. I want to break shit, I don’t break shit. I want to run away, I don’t run away. I don’t want to say I completely discount my feelings, but the controlling idea of my marriage, even when I’m upset, is “What is reasonable and what is fair?”
Is it fair for me to restrict my husband’s relationships based on an emotional impulse to keep him all to myself? I don’t think it is.
Is it fair for him to excoriate me for being honest about feelings I have no control over? Nope.
Would it be fair of me to expect him to not have any negative feelings at all about it? Also, no.
Much like you can create an endless cycle of betrayal and distrust, I think you can also create an endless cycle of mutual understanding and giving the other person a benfit of a doubt. IME the more you make room for your partner’s feelings, the more they are inclined to make room for yours.
Now I want to give some allowance that not everything is an issue of maturity, necessarily. Some people just have different expectations and standards in their own relationships, and maybe two people agree that spending the night at a single friend’s house is crossing a line for their relationship. But there is still plenty of room to demonstrate maturity (or not) in that context just as well. Which is to say, polyamory or even being cool with your hubby crashing on other women’s couches is not inherently more mature than monogamy or having more conservative standards. The maturity part is how you negotiate and manage those standards within the context of whatever you two (three, four) have decided your relationship to be. But ISTM that in order to make a poly relationship last, you have to be really good at frequently and explicitly addressing those issues to the satisfaction of all parties - which probably requires a lot of maturity.