I keep thinking of poly as being new for me, and new for C & my relationship, but in some ways it isn't.
Back before we were married and for the year or so after, we had a good friend, T. We first met him when he was a overly-smart 16 y.o. kid attending college with another of our friends. Eventually he and C became good friends and were in several classes together, and even lived with us for a while (and was the best housemate we've had).
Well, C & T became somewhat more than friends. Their relationship wasn't sexual, exactly, or romantic in the sense of wooing or dating, but it was very erotically charged and very emotionally intense. I remember feeling neglected as C shared more emotional and sexual energy with him than with me (all while I was trying to plan our wedding and find a job), but never really jealous. I didn't want to take away what they had, because it made the girl I loved happy. I just wanted them to admit what was going on go I would have some framework to make my needs known! I also wanted to be a part of it, as I was becoming more and more attracted and attached to T myself. C finally talked to me about her feelings/desires for T because she felt guilty about it, and as much as I told her that a) I already knew and b) it was really OK, I think she still felt guilty.
We wound up spending quite a bit of time together as a group, with lots of late nights talking about nothing and cuddling under blankets, and it was really lovely, and something I still really miss. We did finally all have sex one night (something C & I had fantasied about, and she'd mentioned to him), which was fun if a bit awkward, as neither C nor I really knew what to do with "boy bits" and T wasn't all that much better with "girl bits" and it was all rather spur of the moment. But shortly after that, they both graduated and we all moved to different parts of the country.
He's still family to us, and we both still have a lot of love for him. It's pretty much impossible, but if he were ever to ask, he'd be warmly welcomed into our life and bed. And I wish there was a way to tell him that without seeming creepy.
It's nice, after doing some reading, to be able to look back at that relationship and have some words for what I felt, and to know that I'm not the only person who has had those feelings or those desires. It's also nice to have realize I have "proof of concept", so to speak, in poly love, even if it was very confused at the time.