I don't think they'd have an easy road at all. There would be messy, and Tony, and confusion, and Tony, and clashing, and Tony, and fighting, and... Tony.

I don't think it's impossible to play out the ship at all, I just think that like so many slash pairings out there people tend to warp the characters to fit into a fluffy framework and circumvent all the mess and damage that would be involved in a ship like theirs. People like fluff more than they like acknowledging flaws. Like an anon said above, it's specifically how difficult the relationship would be in reality that makes it interesting to me.