That's why I said that Dean's Sam-protectiveness is the largest obstacle to get over, really: I don't think that the incest itself would bother Dean as much as it would bother Sam, but if he became aware of how much it was troubling Sam he'd back off immediately.

Maybe I just prefer Dean as the initiator because I love Wincesty angst, and Dean is going to be pretty seriously distressed if he realises he was doing something that Sam didn't want. (Sam wouldn't want to hurt Dean either, of course, but I don't think he would handle the realisation quite so badly.)

Basically, screwed-upness is fun.