It is coming out now because you are getting closer to your fantasy becoming a reality. Fantasy has a way of being nice and neat without all the mess to worry about. To use a poor analogy, it is like making sausage, it is easier to enjoy when you don't think about everything that goes into the process.
Being concerned and worried is normal, it is a big step. It sounds like she has some trust issues. She does not trust herself to stay within boundaries. Not just ones the two of you define, but the ones she has in her head, which could be much more narrowly defined. She does not trust that you will help her if she strays. She does not trust that you will not feel the same about her afterwards and possibly how she would feel about you. I have seen people get upset with their spouse for not stopping them from doing something they have both agreed to, it is a set up for failure. You also need to be introspective and analyze if you have given her any reason, no matter how small, to be concerned with your reaction.
You say you have talked for 2 years, but about what? Has it just been about fantasy of the deal or have you gotten down into the nuts an bolts of what could happen, the repercussions, your relationship etc. You really need to be on super solid ground on this before going forward. Getting to real and complete honesty is a very tough.
I would probably put the club on back burner for now. It will, most likely, be there later on, or some club some where will be. But if you do go, stay strictly within the rules, and help each other do so, watch each others back. And I would go with no further plans beyond talking about it.
You guys should communicate more and then do it some more. You say you have been honest with her, but it sounds like she is holding back and not letting you know what is on her mind. Neither of you need to hold back anything.
She sounds as if she might be worried that she thinks she really may get into swinging from a physical stand point and be worried how you will react, not how you say you will react, to her going all out. Or that she will feel less about herself for doing it. You need to explore this in depth.
At the end of the day swinging is an incredible fantasy and that may be your limit. The reality of swinging is not for everyone. And it is certainly not worth sacrificing or damaging your marriage. It should bring you closer and strengthen you marriage, if it does not, then keep it as a fantasy. Regardless of where you end up, there is no rush to get there, take your time and do it right, which may mean not doing it at all.