Why do you draw the line at cheating ?

IN FICTION. Of course. (bad english, you have been warn)

The other day, I was reading a BL with two ML. One is a cheater and everyone was shitting on him in the comments saying how they hope the MC will not end up with him at the end (and let's be real there is no chance of this happening) and that he will go with the other ML and all.

And honestly, it make ask myself why everyone is always so on the edge when cheating is involve in a story.

Because, the other ML on that manwha bordeline SA the MC but like people were talking more about the cheating. And, listen, this is really not to start a discourse on black flag, noncon, but really about cheating.

For me it's just another shape of toxicity that can make a story entertaining. I was entertained as fuck when the MC and that ML were overthere, settling score and all, like that was so messy. But I have the feeling that everyone hate those kinf of interaction IN STORY AGAIN.

Don't get me wrong, I'm a sucker for big love story with characters that love each other to the bones and are like soulmates. But, I love messy stuff too and I really don't understand why the second cheating is involve the whole house is mad.

So why ? Why (in fiction) you draw the line at cheating. And for those who don't, why do you think people do that ?