OK, so I'm not really into the whole Wincest thing. But, your summary got my attention. I mean, it kind of grabs you in a funny (and naughty) way:
"Logically, the only person on the planet Sam should be fucking is Dean."

I love Sam's rules/rationales for it all. And then the explanations are quite humorous. I can almost see him weighing the pros and cons in an Excel spreadsheet on his laptop. I mean, after all, he was studying to be a lawyers - you know he's planning out a strategy and defense for all his thinking!