It sounds like there are two issues here. First, you found out something about him that you would not have expected. He is a complex male human being. Men in many ways are so different than women that it is difficult to really understand them. I think the male aspect and the fact that he is older and very settled had some influence on his feelings. Do you really know what others think and feel always? Could you predict his every thought? This for me at least would be more forgivable.

The second issue is that he tried for many months to persuade you to feel differently and only let it go when he realized that he might lose you. Is he missing something in the marriage? Is he very communicative about his feelings? Perhaps talking about what he wants would help you understand better and you both could come up with a joint solution? I am not necessarily talking about the 3-some issue but more general issues.