I know the majority who hate PASSION won't be persuaded otherwise (because believe me, I've tried), but I do want to clear up a few misnomers from the previous page.
GIORGIO does NOT take up with FOSCA on the rebound from Clara! He dumps Clara because he realizes her compromised affection isn't true love (which the show sees as all-consuming passion). This isn't a matter of interpreting subtext; he says so in the text. (It also isn't fair, since such a passion would require Clara to give up her child, but the show says quite clearly that such love is not about fairness.)
GIORGIO is NOT mentally unbalanced, even if you disagree with his choice. For one thing, he's a dramatic character (and a Romantic one at that); he's not a patient on your couch.
Fosca and Giorgio are very much like opera characters, so musical theater lovers are entitled to dislike them, of course. But is Fosca any more "obsessed" than Gio-Gio San or Lucia?
I find this quote from henrikegermann really interesting:
"If Giorgio were a woman who, having been ultimately rejected by her married male lover, came to love the unattractive, manipulative dying man who was consumed with her and stalked her, because no one had ever loved her like that incorrigible man had, how would we describe her?"
Actually, henrik, I think you'll find that basic plot in any number of plays, films and TV shows. (ESPECIALLY films and TV shows, where the standards for female beauty are so high.) In our culture, we have little to no problem accepting that a beautiful woman will love a physically unattractive man; but we struggle mightily when the genders are reversed. In how many 1980s teen films does the heroine finally realize the hunk is a jerk, but the average guy who loves her obsessively is a gem? But I repeat: Giorgio is NOT rejected by Clara; she simply wants to maintain their affair as it has always been.
The theme in PASSION is not unlike Sondheim's recurring theme of nice v. good. Let's call it "love v. fondness". Fosca loves "purely" because nothing--not dignity, reputation, practical concerns nor even hope of being loved in return--dissuades her. Clara is fond of Giorgio, as he is of her, because she enjoys his company as long as it doesn't upset her well-ordered life.
We may all agree that Fosca and Giorgio are not good role models for modern life and marriage. But that really isn't their job. And myself, I'd hate to settle for a marriage that doesn't contain at least a little of their spark.