I agree it's not that big a deal to some people, but completely disagree with the comment "nor should it be".
There's no 'shoulds' about it. That's way too prescriptive and narrow-minded.
No-one who's in a monogamous relationship where love is involved actually knows what they'd feel like if it happened either, until it does. As in many life situations, theory and reality are two very different things. People very often react differently to how they'd imagined. I've seen that play out so many times in real-life.
The question is about people who sweep it under the carpet and pretend it hasn't happened. For every person in that category for whom it's not that big a deal, I think there are more who do consider it a big deal, but they under-react because of their own trade-offs. Either they've got their own secrets that wouldn't bear scrutiny, they don't love particularly deeply and so the hurt is not that great, they fear being alone, or they don't want to give up things that they value more than the relationship itself - such as material comforts, religious commandments, status or a live-in mother or father for their children. Sometimes all of those things. Of course, this means it's much more likely that it will happen again and the risk of losing those things becomes greater, so it's not a particularly sensible or rational set of trade-offs if they are being made in order to hang on to those things.
I once worked with a woman who thought she was being very pragmatic about her husband's repeated flings. Their relationship was 'open' on his side only and although theoretically she knew she could do the same, she didn't want to. She thought that turning a blind eye and her husband's promise of discretion was the grown-up thing to do and believed the big picture was keeping their family intact. She said the first one hurt the most and after that, each new dalliance hurt less.
Until he fell in love that is and left her after their youngest child had left for university and she was in her fifties. I'll never forget her pain at that - she just hadn't reckoned on him developing feelings for someone else and losing the control she thought she had of her life. Her bitter conclusion was that she should have got out sooner and that her 'pragmatism' had been a massive delusion.