Hmmm, instead of thinking "not hurt their partners", it's more like paying attention to the threshold of pain their partner can take.
True, and by doing that you are taking care of your partner because you don't want them to hurt more than what they can actually can take
I am not really minding the lack of safe words because the point of the story is that the Sub depends everything to the Dom. The Dom is in charge of everything and they are not binded in a contract because they are set up as exclusive partners, like dating with a side of BDSM. It's not how most real people do it and mostly only for those who are experienced but this is fiction so the author chose to combine those together.
The only thing that's missing is the safe word. Every SM play has to have a safe word and besides the main idea of this fetish is not hurting your partner's body, of course pain is involved but partners always pay attention to not hurt their partners hence the use of the safe word.
All in all is a nice manga. This is the first one I read where it depicts the reality of excrement while having anal or better yet the proper way of cleaning yourself before having anal.