Well again, getting back to OP's question of westerners being more upset by infidelity than the French, I'm suggesting it's because westerners are ignorant to how human sexuality really works. The fall from utopia probably hurts more than the fall from reality, but I agree it's a fall nonetheless.
It's like some guys I've known, who come to me all shattered: "I was working 90 hours a week selling life insurance and paying all the bills. Yeah, I was never home and the sex was non-existent but I was a good provider! She actually fucked the garbage man behind my back! How could she?!" To that, all I can say is ...it sucks and, yes it's a betrayal. But like I said earlier: you better go into long term relationships with your eyes wide open. If you're astonished that scenario played out then you went in 100% blind, my dude.
It's reasonable to be upset you didn't get any presents Christmas morning, but also feeling hurt that Santa himself never came down the chimney, that's on you.