Would you consider going over the whole situation--before that night, during that night, and after that night--with a therapist?

In addition to what Ceridwen18 pointed out about heightened emotions, I wonder what role the speaking of your best friend who passed a few years ago played in stoking the intensity of your emotions.

As for reasons for doing it, I don't think there are "reasons" for doing it. I think that the events leading up to the situation made you highly vulnerable. Specifically:

--being drunk
--separation from girlfriends
--emotional time on beach alone with guy, including discussion of late best friend
--screaming argument with sister-in-law
--leaving the room
--choosing to go to the guy's room rather than a girlfriend's room
--finding calming comfort from the guy

None of the above is a reason or excuse for cheating but in my mind these events all compromised your judgment such that you did not think or resist temptation as you normally would. So I think that there would be value in going over each of these events with a therapist in order to understand what was going through your mind and why you acted as you did at each of these steps.

For example:

In looking at this statement, I see you being hard on yourself for being stupid, but not for being drunk. My thought would be to look at the whole situation from the very start, including the decisions that led up to you being drunk and separated from the others.