I guess we were "poly friendly" from the scratch, and way long before even thinking of swinging.
We've allways think the question "who do you love the most, pinapple or light bomb?" doesn't make sense, that a ruler isn't enough to measure love, and we've been very respectfull for each other feelings, and to respect and embrance the love each one of us may feel towards anyone else.
As for me, this mean, I am happy by enjoying what my wife have to offer to me, and I don't need to compare it with what she may be oferring to other people. At least for as long as it fulfills my needs, and since it does, well... everithing is fine.
We've always said "ok, if you have a crush on someone and want to get laid, go ahead, just, avoid telling me something you know could hurt me... because if you do, the more likely is that you wouldn't be doing that TO YOU, but TO ME". In the other hand, and even when having this permission granted, I didn't mess around, nor she did (as far as I know). We supposed (perhpas from the culture and education) that KNOWING about this would be likely to hurt us.
When we started swinging, we realized that knowing didn't hurt, at all. We felt happy to actually know the other was enjoying it and being pleased. In the other hand, the idea of avoiding developing feelings while swinging, even when sound as an advice to avoid unwanted drama, didn't properly fit in our previous mindset (nor we feel affraid of such a drama, meaning we knew anything would be able to damage our bond).
Then we swung with this woman, she tured out to be a great friend and we started hanging up togheter, or even doing things between any two of us without a third one present (including having sex), we all felt ok, we talked a lot about this and we started openly extressing our feelings without any deal, nor any drama.