More info about the affair and the dynamics going on:
(I do find it hard to call it an affair, but I understand that's what it was. For a long time, I bristled when my BS called it an affair. I really bristled when she referred to OW as "the other woman" or my "affair partner". Early on, when I lost my cool, I'd say stuff like "she has a name" or "her name is X!". Yes, I know this is bad.)
The October 2011 Trip:
Before the trip, my BW expressed concerns about me touching OW irl. I am quite ashamed to admit that I insisted that touching was ok because I thought it wasn't romantic. We did make an agreement on what I would do if I felt anything romantic for OW on the trip or if anything happened when BW wasn't with us. I said I would stop whatever it was and tell BW immediately.
On the trip, there were a few times I asked BW if what OW and I were doing was ok. One time, we were sitting on a couch, OW on one side of me and BW on the other side of me, looking at photos we had taken I think, and I asked BW if she was comfortable and if it was ok to hug them both together. She said it was ok.
On another day, the three of us were out on a boat together. It was cold and windy, so BW had gone inside to the heated part of the boat. OW and I were trying to stay warm but enjoy being out on the ocean, so we leaned against each other as I held her hand in my coat pocket. BW came out and saw us and then went back inside. I remember being horrified that I might have gone too far so I went in to talk to her and ask if it was ok. She said it was fine and she was just cold.
She told me later that she felt pressured to say these things were ok. I was upset when I found out because at those times, I was genuinely trying to make sure I was being respectful. Of course, if she had said no, it's quite possible I would have reacted badly.
After the trip, I asked our mutual friend (the poly one, actually, though it was just her, her husband, and her kids who met us one day on the trip) how OW and I looked and she and her husband could tell immediately that something was going on between OW and I. I insisted it wasn't and that we were just "friends who loved each other". (Yes, I know how stupid that sounds now.)
Also after the trip, my BW started calling OW my "girlfriend" and researched polyamory and other such arrangements trying to adapt to the situation. I was frustrated because I did not think it was romantic at all and continued to insist we were "just friends". To be clear, we had touched a lot on the trip (lots of hugs and even a couple backrubs, with BW's "permission"), but since there was no kissing, I guess I was able to fool myself.
We also kept having trouble with time. BW tried to accommodate the relationship, but I kept wanting to spend more time with OW than she was comfortable with and we fought about it.
Also, throughout, though more earlier than later, BW would often read along while I IMed with OW and even offer advice on things we talked about.
At one point in December, I thought I might have felt something close to romantic and began to suspect I was blocking. I realize now that I should have stopped things then, but I used the excuse of wanting to find out what I was blocking before I stopped things to go on the trip alone with OW.
I also suffered extreme jealousy of OW's other friends, even early on. I didn't know why. I thought I was just afraid of losing her as a "best friend" and it was me being silly. Now, I realize I subconsciously saw her as a mate and was reacting as if she was in a relationship with me.
Also, I did not tell OW about BW's requests for us to tone down our relationship. Only in December did I tell her that BW was uncomfortable and not happy with us being more than friends. So at least for awhile, I think, OW had no way of knowing there was a problem. She did, however, continue even after she knew BW and I were having problems. BW asked me not to talk about our marriage problems with OW and I generally complied, however. That doesn't excuse me or OW, though, I know.
After DDay:
One thing BW and I have disagreed on is whether I was replacing her. I felt like my relationship with OW was a new and separate thing, but BW said I was replacing her. She said she felt like I sacrificed her and our marriage in order to feed my relationship with OW.
Hope this info isn't too extraneous... just thought I'd get some more stuff out there.