How I Feel About What Happened:
I acknowledge that we hadn't promised each other exclusivity, but I still think you acted in a selfish manner. You knew that I would be hurt by it, you knew that I wouldn't like it, I explicitly even expressed it, but you didn't care enough how I felt to let that stop you. You said you felt "rotten" about it,
I think it's quite inconsiderate and selfish to: introduve me to your friends, walk hand in hand with me in public, put your rings and necklaces on me, talk to me every day, make plans to come over and cuddle and be intimate, make hints that we're moving towards exclusivity, but then still go to sleep with someone else and lie to me about it. It feels like none of it mattered to you.
Your actions communicate that it was more important for you to sleep with someone else than to consider my feelings about the situation. It feels like it didn't matter to you that what you did hurt me, as long as you got to sleep with someone else.
Why? Why didn't it matter enough to you what we had started building? Why didn't it matter to you what I felt?