i wanna put my thoughts in this too. the way i see it, is an inverbal communication between them. it's said that japanese can read the air, the ability the western did not have. i think consent is written in the air, we the readers from different cultures just can't read it.
Lets not do orientalism here, people everywhere are just people. I think there was no "proper" consent here and I also don't think that's necessarily a problem--both because it's fine to depict situations that are unhealthy or turn out badly, and because you can have a positive story where the characters don't do formal consent properly: you don't need it to have an interesting story, nor to believably have a situation that enters what we might consider BDSM territory that turns out okay.
Proper consent is a safety mechanism that's really helpful to have and use because it's a strong way of ensuring and verifying that a potentially unsafe interaction (like one person hitting another) will follow a safe pattern, which is in turn a way to ensure everyone comes out "okay," with no damage. The reason proper formal consent is good is because it maximizes the chances that the interaction goes well. But you can still have a risky interaction where everyone turns out okay: there is more of a potential for things to go really bad, but that doesn't mean they will. In other words, you can do unsafe stuff like walk too close to a road with headphones on and turn out okay, even though it's not advisable.
Using the constructs of proper, communicated consent is one of the safest ways for things to happen, and might be considered one of the best, but sometimes you don't have those kinds of frameworks, especially if you're not educated on BDSM, which most people aren't.
This is a story about a sadist and masochist who aren't fully sure of themselves, who don't run in those communities, coming together. Depicting what might happen in those situations is a worthy subject of art, and it works better if they don't unrealistically have all the "right", progressive, PC frameworks about consent. You can see how that can result in a scenario that may cause some damage, guilt, fear, but also a connection: hidden, socially unacceptable desires being expressed, satisfied, received by another.
This short manga portrays those feelings beautifully. It works as a piece of art because they did not communicate properly.