Keep in mind, at whatever point you do decide to draw that line people who cross it must be punished by law. If not, then you have not drawn a line at all. Also such a law could only be enforced on commonly owned land(public property, or what I like to call PURGEZONES). It is interesting how people get so in love they fell they can do it in a PURGEZONE; I suppose as proof of dedication to each-other.