The following will not work for everyone. It takes an incredible amount of security in oneself and in one's relationship for this method to work and it is just not to everyone's taste. But you asked how we would deal, so I'm answering how I would deal.
Personally, I don't sweat anything my partners do with other partners so long as it doesn't negatively impact my life. In other words, if they don't bring home an STD or a destructive OSO, whatever makes them happy makes me happy.
1) The reason I can do this is because I do not attach my sense of specialness onto actions or activities. My relationship with Tacit or Datan0de is not special because they do X with me. My relationships with Tacit and Datan0de are special because they are with Tacit and Datan0de.
There is nobody else anywhere in the world exactly like me, so Tacit and Datan0de can do the same things with other people and still have a completely unique experience with that person that does not change in anyway their experiences with me.
Sex with them is still special because our relationship is special simply for being a relationship with unique individuals.
BDSM with them is still special because our relationship is special simply for being in a relationship with unique individuals.
They can top someone, dominate someone, sub to someone, use toys on them, take them to the same restaurants, dance with them, watch the same movies with them, anything they do with that person will be a DIFFERENT experience than doing that same activity with me.
2) Because I love them, I want their happiness above any other feeling for them. If they are happy doing something with another person, and that something isn't actively destructive to either of us, then I am happy for them. I take a great deal of pleasure and joy in their happiness. I am absolutely delighted every time Datan0de tells me of his latest wrestling match with his wife, even though resistance play is one of my HUGE kinks (and a serious issue of trust for me) because I know how much he loves and adores his wife, and their continued healthy relationship makes me happy. I love hearing about Tacit's adventures at public play parties even though a lack of a partner to play with in public has been an ongoing issue for me because I know how much he loves what he does and his continued healthy relationships make me happy.
These are separate issues. My interest in resistance play has nothing at all to do with Datan0de having a resistance scene with someone else. That's a totally unique experience with someone who is different from me. My interest in scening in public is not related to Tacit's public scenes. That's a totally unique experience with people who are different from me. It does not reflect upon the specialness of those times when they each scene with me. Having ongoing relationships with other women where these elements that are important to my relationship with the guys are also important elements in their relationships with other women does not reflect or change the fact that they are *still* important elements with me. Back to point #1, their scenes with other women are DIFFERENT than their scenes with me and their relationships with other women are DIFFERENT than their relationships with me, because I am a different person from those other women. The energy is different, the outcome is different, the details are different, and both my guys see us as different, individual, unique women, whom they happen to love.
3) Doing things with other people is an opportunity for my sweeties to learn something new that can be brought back home to me. They could learn a new technique, or they could learn something about themselves and grow as people. This only enhances my relationship with each of them. Why would I possibly want to turn down an opportunity to make my relationship better simply because the catalyst would come from someone other than me?
I recommend reading the following articles:
http://tacit.livejournal.com/148633.html - Some Thoughts On Being Special
http://www.xeromag.com/fvpolypiano.html - How To Become A Secure Person
http://tacit.livejournal.com/241568.html - But What If He Compares Me To Somebody Else?
http://tacit.livejournal.com/249011.html - Some Evolving Thoughts On Veto