In his essay On Liberty, John Stuart Mill presents the famous harm principle in the following manner: “[…] the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is self-protection. […] The only part of the conduct of anyone, for which he is amenable to society, is that which concerns others. […] Over himself, over his own body and mind, the individual is sovereign.” Hence, there is a (...) distinction between self-regarding and other-regarding acts, and only the latter are subject to moral criticism. However, while all acts are in some way selfregarding, it is not clear if there are any which are exclusively so. There are two additional difficulties. First, the “individual” may not be an individual person; self-determining communities, at least when they have the ability to decide for themselves, are also “individuals” in this sense. Second, it is claimed that groups of acts (activities and practices) have a different kind of justification from single acts. So what are the limits which “others” have in order to protect themselves from what “individuals” (personal or not) do, and what are their rights to do and to protect? If, in the final analysis, protection or defense is a source of justification, what should or must be protected, and why? Where does the demarcation line between self-regarding and other-regarding acts lie? In our age, as in Mill’s, we encounter many situations where such a line is needed, yet is hard to determine or establish. One such example, the case of same-sex marriages, is further explored in this paper. (shrink)