to bring the subject up to your spouse, and I would agree with the others that talking before being stupid and uncaring and cheating on the person you are with is a good sign. Like Gala Girl said, you would do well to find a way to tell him how much it means to you that he felt close enough with you to speak up. Obviously it is hard for you to deal with, but honestly I don't believe that there is a person alive who has never had a least a fleeting thought that was of a sexual nature about somebody other than their spouse, no matter how perfect the relationship is.
It just happens,
maybe not frequently (but for some it is frequent) because we are by design/nature (however you choose to view life) regardless of your religion or lack of of it, sex is THE strongest motivators/pangs/urges whatever name you give it, it is by far the strongest desire that isn't "technically" needed to survive, although it is necessary to survive beyond one generation.
Hunger and thirst are extremely strong feelings, but the way the brain works in the human being hunger and thirst have nothing on intimacy, esp considering food and water is so readily available. Think about it, as strong as the pangs are from hunger and thirst, the reason their strength are what they are is because much more than a day or two without water, and to a lesser extent food, you would die.
Sex is just part of the intimacy equation, but it happens to be the one that spouses who claim they have never ever had at least a fleeting thought involving someone else, are in fact living in denial.
So while ManofDiscovery, I am sure is trying to help, that type of talk can be deleterious. It makes people afraid of the Truth, which between spouses such a fear can mean death of the relationship.
So Murphy, take solace in the fact that you and your spouse just overcame the hardest step of all, being honest with your spouse. Regardless of whether or not it ever progresses beyond just talking about it, the two of you have an opportunity to have your relationship strengthened by a much deeper level of intimacy, through honesty and understanding. It isn't easy, but it is so worth it, and it can be obtained without ever actually 'opening up' your relationship.
I think the most destructive aspect to peoples lives as the world transitions into Truth, is because people do not truly know themselves, so go slow, and realize that as humans, we are extremely emotional beings.
I know I am going to sound crazy, but it isn't just your husband and it isn't just sex, the days are here that the Entire World is transitioning to an Age of Truth and Honesty (They can no longer be denied), and unfortunately it will break many people. You and your spouse can make it through the transition.
It may turn out to be a blessing that you and your spouse learned about real honesty and truth through this method of it manifesting in your marriage, because I believe that truth and honesty will overtake everyone and in all aspects of their life (it's been written about for quite some time, the old world of lies ended 12/21/2012). Most of the world is going to get pushed off of the proverbial Truth Cliff, and because they weren't prepared, it will destroy them.
You and your spouse will have honesty down pat, and will love life, regardless of the aspect of sex.
hang in there, you will likely find yourselves to be counted as one of the lucky ones