
It's not only rape. Abuse ( bullying, beating, forcing the partner through physical intimation) is glorified as 'cute and love' in shounein Ai .
For the readers who flat out admit that they enjoy the kink, it's okay but then there is the horde of readers that, even reading the shit as canon, go out of their way to explain why is excusable, why the 'dominant' partner actually cares very much, etc, etc.
Case in point: The pair He Tian/ Mo Guan Shan in 19 Days. It shouldn't be hard to say that of course it depicts an abusive relationship forcing a pet/owner dynamic by one partner. Try to point out this though. The girls go crazy if you dare to point that that dynamic is not about love but only about a kink of master who will always force his desires on the other person.
For so many yaoi/BL readers who get mad and scream that they know to recognize rape/abuse/unhealthy relationships in fiction, it doesn't sound the case when you bend backwards to explain why it is not.

In some manga I've seen people post comments or make topics about how the rape scene was consensual, even in manga tagged as rape or given a rape warning by the scanlators. I think that's what OP is referring to.

If a manga is labelled rape there shouldn't be an argument about it.

for those who are curious, there is a varied reaction to rape in this manga. and yes, there are those that even try to justify it. http://www.mangago.zone/read-manga/saitou_kun_to_190_cm_no_otoko_2/

one i was reading, lover boy, had a good, decent story going. then they broke up and the guy went for anger/revenge sex. the typical "You owe it to me until i feel better" kind. and they all just justify him being so abusive to his not-at-the-time partner. and i pointed it out over and over again. and they just refused to acknowledge it.
like, his partner was running himself ragged, had problems of his own, was taking sleeping pills, only sleeping 3 hours a night. having nightmares. these kind of issues. the abusive guy knew this, and still treated him like shit and mis-handled him in bed, and talked down on him.
i count that as abuse in my book. but they all disagree. and won't even sympathize with the one suffering the most.

they will go into paragraph after paragraph arguments of bull-shit. of "well he was feeling it", "He was wet", "He looked to be enjoying it", "he consented with his actions"(that's a new one for today). these lists could go on and on, and you can't even argue. because people who see it that way will deny it until the day they die.
it's the cliche, "Well he had feelings for him after the fact". Yes. cause realizing days or weeks later you like them, means it is no longer rape.
i don't mind reading things like this. but it's the complete denial of it i get so mad.
They even have big ass "Rape/abuse" tags, and people STILL deny that shit.

Interestingly (and by that I mean annoyingly), there's topics similar to that in manga outright about abuse, like Killing Stalking.
i've seen so many arguments of late about this one.
Most of us don't even give a rats ass that people enjoy something with rape in it. it's when they deny it or defend it that i think a lot of us get annoyed.
Like, why is that a thing? Can you not read and enjoy it without justifying to yourself that it's rape? You can like that shit. But god, i wish people would stop defending it.
rape is a cliche. we get that. some, like ones by harada can even be fun for physiological reading. but stop trying to defend it as romance and every other overused excuse out there.