From what I've read, you're feeling left out, and you feel that you aren't being heard, and that he's putting her feelings and his feelings for her before you.
It would seem that "they" think that since you're not available, it would be reasonable for them to spend time together.
They are likely stuck in the throes of NRE and see what they're doing as completely reasonable, and you're being unreasonable, irrational and perhaps controlling.
All the while you simply need something from your fiance that he's no longer giving you.
And you want to be nice, you want to make everyone happy so you acquiesce, even if you feel like you are being left out, and left behind.
You need to clearly lay out what you want, what you need and how you feel. It may do no good - they may be so lost in each other that they can't see, despite you clearly laying it out, how you hurt. But you need to try. And hopefully they hear you.
Keep trying to communicate with them. Hopefully they get it sooner rather than later.
There are three sides to every story - yours, mine and the truth.