Well, I feel your pain, as I am currently going through a similar situation myself with my wife. We've always had a great relationship, and we're somehow even closer since talking about opening up to poly (and the sex is somehow better, too...). She's found a beau, yet it's long distance and nothing physical has happened yet, although there are tentative plans. I myself have yet to catch an eye, let alone a heart.
We've been talking about the sex card from the beginning, and have both agreed that if we're going to let each other be their own person in other relationships, setting boundaries for the other person with regards to sex is not fair (nor needed). We both trust each other implicitly, and know neither would do anything that either a) might hurt someone or b) the relationship was not ready for. So, if my wife goes to spend a weekend with her beau, I pretty much can be assured they're going to have sex, and that's ok with me, 'cause it's the right time for it (for them). It's their relationship, not mine (although we both understand I play a part in it implicitly).
All said, for us it comes down to communication. We've discussed all possible outcomes, good or bad, and explored our feelings. We've listened to each other without judging on emotion, and trying to understand where things are and where they would be "if". Therefore, we have a very strong feeling for the context of our lives, and trust that we'll each make decisions that are good (but allow for human fallibility).
Hope this eases your burden, and feel free to ask more questions (both here and with your hubby. Questions are good - if intended well).