Unlikely they agreed upon a safe word when the seme didn't even discuss what activities he'd be doing beforehand. (why would the mangaka include such a detail off screen anyway?) And the uke shows no signs of enjoying the pain. No signs the seme would be aware of anyway. The uke likes being treated kindly, not being hit, gagged, or having an enema forced in him.
I went to reread the chapters and your findings are correct. Akihisa eventually remembered what he did in his drunken state and gave consent before Shizuru continued. If not he probably would have violently protested if he didn't want it.
Secondly. With the second chapter. I think it would be more of Akihisa convincing himself that he didn't want it and also more of how Shizuru wanted to show Akihisa that he could do such things...
I'm not an expert, but rereading brings new perspectives at times.
Actually, I think the problem that a lot of people were having was underlined in your comment. How do we know there wasn't a safe word, felt like he didn't mean it when he said no, pretty sure the seme would know if he meant 'no' versus actual no stop. It's just plain uncertainty.
There would likely have been little to no issues found in this if there HAD been a scene were a safe word was mentioned, but the fact is there wasn't even though like... how many panels would that have taken. And with something this heavy and emotional, it's nice to have a scene depicting that. Without it, the scene can be read as a bit more emotionally manipulative and worrying, like... 'You gave your consent one time soooo, all this is happening to you because you wanted it :D'. It implies his 'no' is no longer valid. Like, you can be an M and not like everything that happens to you. And when it's too much and you find something you hate... bam. Safe word.
And yeah I could pretend they had an agreed upon safe word (and if they did, um seme dude just tell him the safe word when he's sober since he couldn't remember anything. And since he didn't that's more evidence to the fact that... really they probably didn't have one) but a scene or a panel would have been nice so the beginning of the manga wouldn't have been so off putting.
So funny how a lot of you guys claim to be BDSM experts :'D
First of all, he did give his consent - please re-read the end of chapter one.
Second, about the safe word: How do you know there isn't one? The seme wanted the uke's consent first, so maybe they also agreed on a safe word we just don't know about, yet?
Third, I do understand how some people might be upset about chapter two: when the uke said no, the seme gagged him. The thing is: I felt like he didn't really mean it. Remember all those yaoi mangas where the uke, while passionately having sex, still says "no no no, please stop"? Similar vibes here.
I admit, he was afraid of what was going on - but imo, he was afraid because he liked it. I'm pretty sure the seme noticed this - and he'd have stopped if he thought the uke absolutely hated it.
The point of this manga is probably how the seme makes the uke feel pleasure from pain since he's new to that stuff and never knew all those sensentions. He saw his "potential" and is awakening it now.