I'd say some imaginative rehearsing would go a long way to figuring out how things would feel. Spend time imagining what would happen and see how that feels.
Using me as a reference point, as a hetero male, I would have to imagine the ladies with whom I am and/or want to be involved interacting with another male SO. Picture them greeting each other with hugs and smooches, showing affection as they sit and talk. Perhaps even imagine the ladies getting nekkid and flashing "fuck me" eyes at other men. The ladies going to parties or dinner or movies or plays or whatever *without me* and *with somebody else*.
Then sort through all of the feelings those imaginings generate. Figure out what bothers me and why.
Of course, to continue it, then I could imagine her when she then spends time with me feeling so much more fulfilled by having other loves. How her expanding joy will make things so much better when I'm with her. And then how much better I'll feel not having to try to fulfill all of her relationship needs. Stuff like that.
There's much to recommend that sort of mental rehearsal as a form of preparation.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.