
You know people accept rape stories because of how mangaka romanticizes rape in stories, thats how its easy to read one and the readers easily accepts the rapist. There are manga where rape isnt romanticized at all and this will be where you'll find same readers criticing, rejecting that forceful action.
Its not that people are concerned about cheating but accepting rape. In this manga, its because the seme is so nice that people are throwing criticism. A while back i read a manga and the seme cheated on his fiancé for the uke, but in there cheating was romanticized by the mangaka. The other party is shown to be a vilain towards the couple by threatening them. Readers there for a single moment didnt criticise how the seme was a cheater rather the whole comment section was sending death threats towards the fiancé for threatening the uke. They even justify this as the seme finding true love while the fiancé was just about an arranged marriage
So its all about how the mangaka portrays the action.

It's soo true! But it's saddening that we just read the way the author wants it, instead of thinking about the stories in a more critical way...
For me the perfect example is hidoku shinaide. I love the manga sooooo much, but the starting is still wrong. So I find myself conflicted...
Or there is a story of a homophobic senpai who gets drugged and raped by the seme, and I found myself siding with the seme, which looking back disgustes me. And like you said, I wasn't siding with the rapist, because I love rape, but because of the way how the story was portrayed. I'm sure non of the readers would have sympathized with the seme, if it would have been written from ukes pov...
I love how everyone is so concerned about cheating but when it comes to rape oohh yeah it’s not such a big deal it’s fAnTaSyYy