I have never been on the receiving end, but I have been the friend that a significant other gravitated towards. My best friend in high school was a girl (I'm a guy) and she had a boyfriend who I knew somewhat but not well at all. Well, we started hanging out as a group, but my friend had times where she was busy but neither of us was, so we began to hang out without her. We found that we had tons of similar interests, and it got to the point where me and the guy would be hanging out every day. My friend told me she was feeling left out and I felt absolutely horrible. I never thought about the fact that I was taking time away from her and her boyfriend; I was focusing on my new friend and all of the fun we were having hanging out. Once she made it clear that she felt left out, we redoubled our efforts to include her in everything, and as corny as it sounds we all became much closer to each other after that.
I think everything will go back to normal over time once the newness wears off. Their friendship is still new and exciting to them now, and I'm sure they never even thought about the fact that they're excluding you from anything. Tell them you're feeling left out and I think they will understand.