The deal with Craigslist and "humiliating" himself etc. by have sexual contact with a male sounds like a big, huge, quack to me. He says he was feeling so weak and gross and unmanly that he did this as a punishment to himself.
Interesting that he has that much introspection since most people don't have a freaking clue why they are attracted to or interested in certain things sexually. In other words, I doubt this explanation.
I do agree that someone have fantasies about same sex partners, or even having same sex experiences does not make that person gay. The question here is your partner has a history of sex being difficult with you, but apparently not when fantasizing about submissive sex with a man. I am not implying he is gay, I don't know. He may be. He may not be. He has issues though, and its above your pay grade to figure out what they are.
I would put all that to the side and focus on what you know to be true. Your partner cheated, and has submissive fantasies with men that he trolls on Craigslist. You need to figure out what YOU feel about this rather than what is motivating HIM to do it. We can spend years lost in psychoanalyzing the actions of a person and excusing their actions due to some "mental" explanation.
The one thing I can promise is this behavior is not due to an addiction. Addictions can lower inhibitions, but they don't create whats not already there.