My fiancé (27) & I (22) have been together for ~7 years. We had a threesome over a year ago with a woman that he worked with. It was enjoyable in the moment for all of us, but it was awkward & we didn’t handle it very smoothly. He actually had sex with her before the three of us did anything (or before I really talked to her about much of anything) which he told me about and I didn’t hold against him because it was a result of unclear communication between the two of us. In the end, we learned a lot & it made me want an open relationship even more. The issue is that my partner is only interested in women for us, together or separate. I identify as bi & have a huge crush on 1 of our mutual friends who is a guy. My fiancé knows about the crush but still feels weird about opening up to me having relationship with another man. He says he just has a harder time trusting men, but I’m worried it could be stemming from jealousy or lack of self confidence or something. I don’t want to do anything that would hurt my current partner, I love him deeply, but I also reallyyyyyy like this other guy and want to talk to him about how I feel, but again, don’t want to overstep any boundaries or hurt feelings or miscommunicate. Were also definitely NOT out about being polyam-curious (I think that’s the right term to use here).
Your first step is to talk to your fiancé about this. It's pretty common for men to feel this way about female partners - that they're okay with their partners dating women, but not other men. Within the polyamorous community, this is often referred to as the "One Dick Rule" or "One Penis Policy."* The One Dick Rule, though common, is garbage, and you are correct that it usually comes from hangups that deserve to be interrogated.
Talk to your partner about this conviction he has that men are "less trustworthy." What, specifically, is he concerned about? This isn't about him convincing you that his rule is valid and should be followed; it's about figuring out how to help him let go of these hangups. Let him know that as far as you're concerned, as a bisexual woman, being allowed to date other people does include men, and you want to figure out how to work that out.
Say that you've been thinking seriously about having an open relationship ever since the threesome, and you want to talk about whether that could work between you, and if so, what it would look like. Be clear and honest about what you want. Only after you two figure out what your relationship arrangement will look like is the right time to come out in your social circles as polyam-curious and willing to date and/or sleep with other people.
I also want to add a note of caution. Keep in mind that you've been dating this man since you were fifteen and he was twenty. This creates some vulnerabilities that you should be aware of. With that kind of age gap, the older partner often gets to determine the other partner's sense of what is normal, so you need to be very careful about that. Don't take his word for it that something he did was unilaterally okay, or that something you did makes his behavior consequence-free, or that his preferences are objective reflections of reality.
It's a red flag for me that he slept with someone essentially without your permission or without working it out with you first, but you're not allowed to be upset about that because it was a "miscommunication." Now you're worried about another "misunderstanding" with regards to these new steps toward opening the relationship. It's not okay that these "miscommunications" seem to fall on you - it's your responsibility to avoid and prevent them, but if they do happen, they absolve him of responsibility.
You need to figure out how to keep yourself safe from this sort of framing. Both of you are responsible for managing your communication. Neither of you are immune from consequences even if you genuinely had a miscommunication. Both of you are entitled to your feelings about things, even if the other person insists that your feelings are the result of a miscommunication. You two need to figure out how to prevent these miscommunications, and if they do happen, how to navigate and heal from the fallout of that miscommunication, rather than brushing it off. That's the first step - clear, open, honest, and safe communication.
*Side note: the last time I used the phrase One Dick Rule/One Penis Policy, I got a message saying that it was transphobic to equate being a man with having a dick. In general, it is true that this isn't a great assumption to make. In this case, however, I do feel that - to get a bit Freudian here - it is in large part the 'threat' of the 'phallus' that creates the anxiety. One wonders how many men who have an ODR in place would 'allow' their partner to date or sleep with a man without a penis, and what that says about their conception of maleness and sexuality. Also, I didn't invent the term, and want to best equip my readers to get more information, and One Dick Rule/One Penis Policy will be more searchable than One Male Person Rule.