Wow, I can't believe how much reading your replies has helped me! In answer to some of your questions:
They will not be monogamously married, and they both told me that. They say that nothing will change, but I think what Tonberry said about feeling lesser makes a lot of sense. I hadn't been able to put my finger on it, but that is exactly what I feel. Their bond is stronger now and in turn makes mine feel weaker. It feels good to be able to put it into words!
I have not confronted them about hiding events, only because they know I have a hard time talking about it, and I think they were doing it to try to protect me. But you are right, I need to tell them to be open with everything otherwise it's worse. And another reason I won't be invited to the party is she is kind of territorial, I wasn't invited to her b-day because she wants B to be just hers for the night, which I understand. It's her day.
I don't have any other relationships going on at the moment, and that definitely has something to do with it. In all honesty, this is my first poly relationship, but I think I tend more towards monogamy. I don't feel the need to go out and date others, but I think I need to.
They decided to get married after her secondary broke up with her. It had never been mentioned before that. And I think that's another thing that makes it hard for me, he originally didn't like the idea, and I feel like he was talked into it. He admitted that he wasn't on board at first until she explained her reasons. She and I do like each other but are very different, so it's hard to understand each other sometimes.
Thank you both so much for your replies, it did help me figure some things out that I need to discuss with him. I don't think he really understands why this is so hard for me, but now I can let him know that I think it's because I feel like their bond is so much stronger and mine is lessened. I think that will help him get it. You guys rock!