Instead, I said, "Right - and it doesn't work because it doesn't take societal mechanisms into account."
She was quick to cover the frown, but it looked like I'd stepped in it again. What had we been talking about? She had been ranting about someone who had defended rape as a biological strategy... and my response wasn't what she was looking for. 
Obviously, I did a horrible job explaining myself then, so I'll try again now. It's important, because we are supposed to be finding the reality of things, not just what we want to see. This example is a perfect one to show the fallacies of purely biologically-driven arguments.
Regardless of how one feels about it, rape exists in nature. Whether it's between ducks (and the configuration wars between the complexity of female and male sexual organs) or other animals, rape and cuckolding occur in nature. Both of these persist, quite probably, because they are one possible reproductive strategy. These things - while distasteful to us - aren't really arguable.
The problem comes when you make that leap to people.
Behaviorists had the same problem; Skinner frequently used experiments with pigeons to extrapolate out to people. We still do it - using animal models in medical testing, or rhesus monkeys for psychological testing. And there's a great big fallacy involved there.
Oh, we're animals, all right. I don't claim any special kind of biological status for us. What's being overlooked is all that messy social science stuff. The things that biologists like to claim isn't "real science". Remember, environment including social factors and society actively alters biology. The thymus literally changes due to stress. Our brains literally re-wire due to things we learn and experience. The concept of "memes" pays some homage to this - but it is real. That is, an idea you get now literally changes your brain... and you can pass that biological change on to others through social interaction.
This is what essentialist, sociobiological, or purely biological models miss. They're right when they say that cuckolding and rape should be a successful reproductive strategy... but they don't make explicit the given condition that it would only work if there were no social element to our species. Our society, our ability to be social has added layer after layer of extrapolation on top of the basic biological "drives". It has done so to the point where they are essentially irrelevant - at least insofar as having explanatory power. Our social forces - societal valuations of women, for example - play so much stronger a role as to make any biological explanation irrelevant. Bob didn't get heart disease because his dad did - he got heart disease because he smoked, ate grease with every meal, didn't exercise, poured salt on everything... you get the idea. Hell, maybe Bob just got heart disease because he had a stressful job.
Our society has a hugely (and appropriate, IMHO) negative view towards rapists and cuckolders. Being hunted down and ... well, let's just say that it's not a viable strategy for survival if I had anything to say about it... and even when I'm not in a vengeful mood, our societies simply make it an unviable strategy for a social species. Further, humans are neither always fertile nor do they openly display when they're fertile, which would make this risky behavior even less likely to pay off.
So we've covered how biological urges aren't proximal reasons, and how, even if it were, it'd be a crappy strategy. The whole excuse is a bankrupt argument. Therefore:
To try to excuse rape via biology tells you far more about the person making the excuse than about the world we live in. Such excuses do a disservice to both sociology and to biology by making each look stupid to the other. Further, as conscious (and therefore empathetic) beings , rape becomes a particularly cruel crime, and perhaps a clear indicator of being a non-conscious entity. (or philosophical zombie).
So go ahead. Make the argument. Try to defend rapists.
It makes it easier for us to identify the zombies.
 I have since found that I misread her expression. Still, the argument's worth spelling out.
 See my bits about Mead from last year on this. Or better yet, go read Mind, Self, and Society yourself.