"Any reasonable person should be able to tell when someone is uncomfortable" - bzzzt! This implies that they display the discomfort instead of internalizing, brooding and acting on it later. The dongle incident is a good example of your statement not working.
The victim in the dongle incident was unusually sensitive. That is why a key element in such cases is that the personal has to externalize their discomfort and tell the perpetrator that they are uncomfortable in the situation. If someone is unusually sensitive then the onus is on them to let people know. If you're doing or saying something that is questionable, it should be obvious, based on the person's reaction, whether you should stop.