I hope it's alright if I add my own new success-story
A little back-story: I have been attracted to this wonderful lady since I met her a couple years ago, but didn't actually begin forming a friendship with her until last year when she joined our family as my brother-in-law's girlfriend. I had been very upfront with my Life Partner/husband about my attraction to her from the moment I met her, and had also been even more transparent with my bro-in-law since we were both so head-over-heals for her. Bro-in-law and I would even spend time together extolling her virtues when she wasn't around!
After a year of feeling attracted to her (but giving them time to explore their own relationship and become comfortable with each other) I finally took my chances (and maybe a couple swigs of beer for confidence!) and stepped out on a limb. I invited bro-in-law's girlfriend... Let's give her a name, Miss M., over to cut my hair - sneaky, I know! I am a sneaky bastard
Anyway, Miss M. and I had us some girly-time and she cut and styled my hair, and afterword with much hand-wringing and a few deep breaths I proposed a polyamorous relationship with her. We both talked to our primaries about it that night, and explained everything separately.
A few days later we met together for "girl time" while our primaries - two brothers - got to spend time together upstairs bonding over video games. It was so comforting - so relieving - so invigorating! - just to know that while Miss M. and I were exploring each other we had the full support of our life partners and family just a story above us! Of course the physical exploration was divine, but the afterglow which we got to share with our life partners was somehow even better! Compersion is absolutely the best thing ever, and I can't believe so many people live with jealousy instead of this emotion.
Since then I have gotten the chance to talk to my Life Partner and even bro-in-law about it personally, and we've even bumped fists a couple times and given each other nods and knowing winks! Bro-in-law has offered suggestions and advice for my next adventure with Miss M. - which I can't wait to utilize! Bro-in-law and my Life Partner (being brothers) are incredibly close and talk about literally everything
together, and so I get to hear through the family grape-vine just how much Miss M. enjoyed herself! My compersion toward Miss M. and bro-in-law has only grown exponentially, and now when I see them nuzzling and cuddling together I can't help but smile to myself, knowing how the love I have shared with Miss M. has only helped cultivate and grow her love and respect for bro-in-law and my love and respect for my Life Partner - if only for the communication and respect shared between the four of us.