Polygraph "tests" are typically scored based on a comparison of a person's reactions to relevant questions (e.g., "Since your marriage have you had sexual relations with anyone other than your spouse?") to his/her reactions to so-called "control" questions that are intended to provoke a response even in people who are truthfully answering the relevant questions (e.g., "Since your marriage, did you ever think about having sexual relations with anyone other than your spouse?"). The person being "tested" is not supposed to know about the distinction between these two classes of questions, and all are presented as being relevant. The general public is not supposed to know about the distinction either, which is no doubt why the polygrapher you hired didn't mention it.

If a person reacts more strongly to the relevant questions than to the "control" questions, he fails. Conversely, if the person reacts more strongly to the "control" questions, he passes. 

If your husband indeed reacted only slightly to one instance of a relevant question that was asked three times, then the polygrapher's assertion that he passed is quite plausible. But you should be aware that the entire polygraph procedure is without scientific basis, as you will find explained in Chapter 1 of The Lie Behind the Lie Detector.

The answers you seek are not to be found in the reading of polygraph charts. Rather than considering a re-test, you and your husband might be better advised to consider marriage counseling and working together to build openess and trust in your relationship.

Let us know how things turn out. I am sorry you had to go through the test. If he didn't pass, it would affect your relationship in a negative way.

I kissed a married guy, and I felt bad about it. What I learned from that though, is that a lot of men are vulnerable.

Rather than harping on the past, and what might have happened, take time to make a better future. Women who take married men are stupid, because sooner or later they will get re-acquainted with the ripples of the thrown pebble in the pond.

Take care and let us know how things turn out. We are behind you. Don't give up too easily. Being single/divored sucks.
Nina