Thanks to Daily Beast’s Marlow Stern for the tip to this Hollywood Reporter article on Nicholas Roeg’s Don’t Look Now — as to the question of whether Julie Christie and Donald Sutherland actually did it on camera:
Well, Peter Bart unequivocally says they did.
I got my hands on a galley copy of the former¬†Variety editor’s forthcoming book,¬†Infamous Players: A Tale of Movies, the Mob, (and Sex). In the book,¬†Bart tells a funny, profane, revealing anecdote about visiting the Venice set on that “auspicious day.” At the time, Bart was a Paramount executive and, according to the new book, Roeg told him as he arrived on set, “Good day to come by.”
The whole piece is a must-read, as Warren Beatty throws a fit.  
A great cinematic sex scene isn’t necessarily explicit. ¬†Hell, that’s why we have porn. ¬†Cinematic sex is a whole different thing. ¬†I’m sure everyone has their own idea of what they look for in a good sex scene. There aren’t many directors who really go there, certainly not since the 1970s closed its doors. ¬†It seems like all of the best sex scenes were in movies from decades ago.
