I've been noticing some comments are being accusatory that we are 'freaking out' only because it's the man being raped here.
I wouldn't dream of speaking for everyone, but I, for one, don't like it either way (unless it's explicitly what I'm looking for in an 18+ moment - MAJOR EDIT- add FICTIONAL to this please, and then [like getting too drunk] not liking it afterward...).
I didn't like it when Isekai character who became isekaied because he found a website that offered to do so finds a slave he was very attracted to, managed to purchase her before she was bought by someone else, and then goes "hey, I 'own' her now, this is my right, right?" (regardless of their relationship thereafter it was still rape as she didn't consent). This is as opposed to that other Isekai where isekaied (instead of going to get a job) character buys a slave because another (noble who slave doesn't like) was going to buy here, builds a relationship, and then has sex with her. Or Isekai where the main character is summoned and a girl who hasn't paid her bills is about to be enslaved, so instead of the bill collectors she selects random person walking by and says "hey, buy me" -- and enslaved noble knight is being tormented by a demon and is saved by same main character, who proceeds to buy her after saving her -- and sisters whose village is saved by main character becomes his followers(?)/slaves(?)... none of which have sex with him because he doesn't bother to ask about contraception (although he still sexually assaults them (with tentacle magic), yet all but the youngest sister, who is left alone, are ok with it because they were all 'saved' by the MC). Or isekai where the main character is summoned and get separated from the other summoned ones and I don't actually remember how he gets his harem, but probably from saving all of them (except for the god of void magic) and builds a relationship with them. Or the best of this argument: isekai where we don't actually learn how or why the main character was isekaied (unless we find and read the light novel) and he proceeds to have sex with everything that moves... ok, that's an exaggeration, but he does get married to 11+ women who (mostly, like two or four of them initially had issues with there being other wives, but were "persuaded" via sex. Yeah... this manga brought the main character who actually asks the important question: "am I just a convenient (vibrator equivalent)?")
Point is that in these circumstances the main character builds a relationship with the females. Their worlds have polygamy and the women accept (for the most part) that the others exist in the relationship.
As for those whining about harem manga where the main character is passive? Well, THIS is a perfect example of WHY they may take that stance... if the manga is set in modern Japan then having sex with ONE of the women/teenage girls would destroy the relationship with ALL OTHERs (or worse... I'm not particularly fond of yandere...) This is one reason why I don't like that they use the word "Harem" for what should be "love polygon" since if it were really a harem then they would all be in an actual relationship (marriage or otherwise) as one unit and not all potential marriage/relationship candidates...
I'm considering dropping this manga, not because of the rape, but because of the completely f***ing uncalled for rape and the completely idiotic followup and lack of reaction/suspicion for the circumstances that could have easily fu**ing been avoided if the female lead was just a bit more patient. If he reacts this way to what any idiot with half a brain should be able to determine was a setup then if she'd been a little more persistent (without going overboard) he would have caved and built a more intimate relationship with her... This simply turned rape into a convenient plot device to instantly move the story into "we're a couple, f'ing deal with it" mode (and if the spoilers are to be believed, give him an 'easy way' to add more women to his harem). There is/maybe no fallout, no consequences, no damage from how this was done... and that's what I don't like.
As I said earlier I do read revenge manga (hero is betrayed and the entire manga is about his/her revenge on the people who betrayed). Hell, I read some fairly stupid or insane things... but if there is no justification for an act that the author/mangaka makes... then I get angry... and if that continues I have to drop the manga because I don't want to get mad (to put it gently) at something I read to enjoy for entertainment.
TLDR: No, I'm not angry that its rape against a man (that's actually refreshing if it can ever be called such). I am angry because it is such clearly premeditated and useless rape. It is purely a convenience to hasten the story. That's unfair to everyone involved, including the girl, who (should) miss out on building a good (healthy) relationship.
P.S. I wanted to emphasize more words (italicize, bold, underline), but can't be bothered to reread it right now to do so with the above buttons...