Cant remember who it was exactly.... but explorers in the Pacific found several Islander cultures where it was normal for visitors to be presented with wives and daughters in order to make them feel welcome and comfortable.
What the Islander girls thought of this practice is unrecorded... but it would seem wrong to apply the predjudices of our own culture on them.. perhaps to them it was a hell of a night out and a wonderful break from routine
My point, basically, is that our own (Western) morals are merely a holdover from an earlier time, where sex is concerned, and merely a cultural thing rather than instinctual. I dont believe in monogamy (unless you both WANT to be monogamous) or anything like that. Christian values (and the vast majority of our sexual mores result from a long bondage under the christian religion) have no real place anymore now that people can control birth.
Goofyfish was asking which, if any, of our morals are fundamental to us as a species.
My belief is that there are none... any crime can be explained away if you consider Darwinian law.
The only exception I can think of to this point is that, while rape in ancient times may have been a way of propagating the species, in modern times it tends to be more of a power/domination thing and be a mental rather than physical attack. I cant think of any justifiable reason for such behaviour to exist.