Domestic violence is an obvious line to set, but are you really going to cut a good friend off for not repaying a loan? I would argue that the former is a net good (protects people from abuse) while the latter is a net harm (causes people to abandon their friends when times are bad, without helping to get repaid). I would also argue that most lines fall into the latter category.
Setting lines to protect moral, philosophical, or political beliefs seems even worse, because it's preventing you from changing your mind about those things. Once you've set a line, you can't adapt to change as easily. Having criteria for changing your mind is a band-aid over the problem, because those criteria are set by you as you are today, and they're biased by your current beliefs.