Well, TP, pretty much handled it.
Speaking as a former girlfriend of a married couple, now the girlfriend of only one partner of that couple, it should be completely understandable that a level of comfort has to be developed and your comfort level, obviously, is not on the same track as the other two. Which is fine. Everyone moves at their own pace and they will just have to except that. I had a similar situation with my partners early on. I am of the belief that if you are sure of what you want then you open yourself up and let things flow naturally. Welll, my natural flow is on a faster track than either of theirs so where I was open and willing, they were expressing some resistance. Communication, communication, communication. I cannot express this enough. It really helped us through those periods of our relationship and continues to allow us to explore and grow with each other.
I hope it all sorts out.