I only have direct experience with a friend of mine starting one up, one I had strange dynamics with (basically a one-way attraction that we were both very aware of). Still, we were close and confidantes. It was painful for me, knowing I'd been rejected and trying to put my true feelings aside (and to be honest, he did the very human thing and used it to his advantage when my attention was valuable to him, which also sucked … but understandable).
We went through a strained period when he started to date someone (we'd both been single miserable fucks for over a year), but I was making every attempt to really let go of him and come to terms with it. That became doubly more difficult when he officially introduced a THIRD person into their relationship. I could never stop thinking that I had been rejected not once, but twice. It's hard to describe my sentiment ... and yes, I know that's making it all about me, but it did help give me the strength I needed to officially move on. It's not like we ever had real drama or fought or anything, we just talk less now and there's some distance.
So, that's one thing to consider -- it's not a simple thing to explain to others in your life, particularly those you might have a history with. I'd go so far as to say it's a lot more complicated to deal with in terms of whether or not the people close to you will be aware of what's "really going on" than the simple act of coming out and introducing a monogamous same-sex partner. Comparatively, that's easy.
As for myself, I used to believe that I would consider it (I even romanticized the idea, as I'm prone to infatuation and the idea of being in DoubleXLove was romantic and exciting to me). That was when I was young and romantic and horny. I'm still all three of those things ... but I also am now better acquainted with myself and I understand how incredibly jealous I'd be throughout the whole thing, and how difficult striking a true equal balance between all three people would wind up being.
It's so easy to lose that balance and hurt feelings to develop; the moment anyone gets the impression that time was spent or stuff was said behind their back, and they feel left out ... it's fucked.
Do what you want and report back, we're all ears, just saying ... understand what you're getting yourself into, and be honest with yourself about what you're expecting to get out of it. Your chances of success and long-termedness is pretty small.
