I find that in these situations it's useful to consider what I would cause to happen if I acted on such a mere crush--what I would think of myself, whether the wife would break down and break my friend's heart, causing the disintegration of a household, etc. For me, that can go a long way toward moving from active desire to academic attraction, which is comfortable and totally acceptable. Until they suggest a threesome, but I think that only happens in movies (and supposedly raises its own problems).