Then you have got to bring it all out in the open with her COMPLETELY. Voice your desires and voice your concerns in the same kind of way that you would talk about any other major life decision.
You mention that she said(half jokingly) that it's all up to you. What exactly does that mean? She doesn't want to be a part of picking the guy? I think it's important, especially starting out, that you are both in 100% which means she is directly involved in the decisions, has input and veto power. I'd be afraid that she would pull the same, "I can't believe you did it" line. By having her involved she can back out at any time and not put you through what you went through before to get back to this point. Can you live with her decision?
Some people just aren't cut out for the lifestyle. It seems like there is a recurring theme on this board where fantasy and reality don't intersect. Some people can separate and compartmentalize the emotions of love, lust, desire, excitement, jealousy, compersion, etc. and are able to deal with each on its own. Other's just aren't wired that way. It's one thing to talk about the fantasy of it, it's totally different to see her actually go down on another guy or watch her fuck another guy and enjoy it for what it is or for her to see you do it. You have to make sure it's something she truly wants to do.