What I'm worried about is either being completely ostricized for being "sexually deviant"...
If they ostracise you for this, it would be a bit hypocritical I think! It would be very unfair to judge you for having feelings for them, and for suggesting an open, ethical way of moving forward with those feelings. If you lost their friendship over that, I'd say - their loss (sorry to be blunt. I obviously don't know the people involved. Just my immediate reaction)
... or either one of them being angry for me coming clean about what was said/done by either of them!
As you say, you may not have to "come clean" about this.
You could have the conversation about your feelings
, and your proposal
while leaving it to them to come clean to each other about the history of this.
For example, you could say: "There's something I want to talk with both of you about. Firstly, I value our friendship and my main concern is to be able to continue to have a good friendship with you. However, I feel attracted to both of you... I know this is unconventional but I wanted to put my feelings out in the open. I've been seriously wondering about the possibility that you both may be interested in pursuing an open relationship with me. This is obviously something that you would need to be sincerely okay with, and I don't need you to say anything either way now. Please think about this and talk with each other about this, when I'm not around. Then later we can talk again. It's okay if this is not what you would be into. I trust we can still be friends. I just had to ask, because of the way I'm feeling. I would prefer to have any further discussions with both of you together, so I can feel confident that we're all on the same page."
Sorry, I know that was long... When I have these kinds of conversations, I tend to be quite verbose!
If you decide to go down this path, no doubt you'll find your own words.
I'd say, if things go well from there, you'd need to make sure very soon into it that everything had been revealed. You don't want to start off with unspoken secrets.