@Spock thanks for sharing your story. I can understand that she would be upset if you hit on her sister or cheat on her - but I'm sure that you could have searched out other options. I was kind of you to refrain from other options as well for a while, it sounds like it was really really rough.
As far why I am exploring poly, well we started poly, I met him when he was still married. He has always said he was poly, I can't change the game this far into it. I knew he was poly and he knew I was pretty mono. We both took a risk that we could weather the differences. I told him before we moved in together that I could not handle watching him fall in love with someone, I would rather gouge my eye out. He had told me he wasn't looking for another relationship, to play and fuck yes but nothing deep. And now "oops" he is in relationship. He hasnít said he is in love but if you hang out in a tattoo shop, eventually you will get a tattoo.
I always challenged him, how can you think you can do x,y,z and not expect to fall into love with a nice person? Funny, people may think that mono means you think there is one and only. Actually I know how easy it is to fall in love, when people share themselves it is beautiful and tender so of course more will follow duh! The difference is the choices to stay focused and he didn't choose it. And he is way more apt to fall in love; he really does have a big heart.
I just canít be that person that says ďno you canít do thatĒ because I donít want to me be that controlling and it doesnít really help. I donít want him to not do things because I stop him but because he makes a mature decision which supports the future we envisioned. But he would in insist that what he is doing has absolutely no impact on our future together. And I would insist that when heís busy fucking around heís not doing other things. He would respond that even when Iím working hard, I still need to have fun. He has a point, I tell to be all work and no play and he leans the other way. Anyway we are 15 years apart in age and with very different experiences and those differences are starting to have an impact on me.
Me: mono, 52, serving S in a PE relationship for six years, living together since April 2012
S: has been poly since he can remember, 37
A: His new girl, under consideration