With respect, I think it's an error to judge the characters of PASSION as if they were guests on a very special episode of MAURY.
For example, the point isn't that Clara is "unfaithful" to Giorgio--she's married to someone else, after all. The point is that she is practical. She will leave her husband to be with Giorgio, but only when her child is grown and she won't lose custody in a divorce. Now realistically, this is a perfectly reasonable, even admirable position for a mother to take. Who among us would blame her?
But Giorgio's response is that practicality is incompatible with passionate love. True love--he has learned from Fosca--is rash, immoral, beyond rational calculation. Of course he's being unfair to Clara, but that's a realistic concern and outside the argument of the play. The play's argument is that TRUE love eclipses everyday concerns such as etiquette and a balance of personal power.
Moreover, I think the OP makes the mistake of seeing the entire play from Fosca's point of view. Giorgio isn't "flaunting" his happiness to hurt Fosca; he's merely discussing his life with someone who doesn't yet seem to him a potential romantic option.
In PASSION, love isn't "nice" or pleasurable or even satisfying, it's an obsession, unattractive and uncomfortable, particularly when it is unrequited.