That's because there aren't any general rules. Even with one person, the boundaries shift depending on mood and circumstances and whether other people are around. It doesn't help that some people aren't good at setting their own clear boundaries and they can end up having regrets afterward if they decide that they are no longer comfortable if things got too intense. A big part of it is learning to set and hold your own boundaries. That makes it easier to understand how other people are doing it, and the challenges they face as they also adjust to changing circumstances.