No offense meant, if you've posted on the subject before I didn't recall, and I interpreted what you wrote above based on what that phrase would mean to me. "Fling" is a word I associate with casual fun only, not depth, but I get that you're saying you see the potential for it.
I agree that there's no cause for judging, but as for how to make it feel more comfortable I'm just not sure. I think I would eventually get really bummed out if I had to be a secret to someone's partner on an ongoing basis and there were real feelings involved for me. If you're a secret to someone's partner you have to be sequestered from the rest of their life as well, you can't openly get to know their best friends or family, can't drop by for a few days, can't send gifts, etc., without risking the secret. Fine (though perhaps less than ideal) for a fun sexual friend, hella sad-making (to me) for a partner.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.