is a trespass into the body of another: penetration of his anatomical boundary without his consent. We deem rape to be a heinous crime because the victim's right to the control of his own body, derived directly from his right to life, is sacrosanct. To one who concedes that sovereignty -- a nation's right to exist, independent of control by other nations -- must include the right to defend a national border, the parallel to invasion is obvious.
Inversely, a nation denied the right to defend its borders against uninvited intruders has been denied sovereignty. By the standards of the Westphalian nation-state system, it's not a nation at all. It's a No-Man's-Land, wherein the "law" is whatever the strongest force in any particular locale deems it to be.
It's a helpless victim of rape.