Not a male but if you want more replies you could post this in the Love section of the forum too.
I would think it is a form of cheating, emotional with the possibility of intending it to become physical on one or both peoples part. I think any sexual talk with someone NOT your spouse or significant other is cheating and wrong, imo. I guess it is how you see it that matters the most. If he was bored why didn't he do projects around the house, take you on a trip, take up a sport? Bored is an excuse to make you stop questioning him and saying he was upset with you is also another way to pass his guilt onto you, and it is not fair. He did it, on his own, was his choice so he should own up to his own actions and stop passing fault to you or other things. If he can't do that, he will probably continue his behavior because he hasn't accepted any real blame onto himself.
“The world would be a nicer place if everyone had the ability to love as unconditionally as a dog.”