The rationalist may not enjoy admitting to the biological evolutionary naturalistic roots of what he concludes is "rational." A good analogy is how Spock was portrayed as viewing the world: logical until mating season came around, but even then his actions were in the end logical to him.
It's is indeed rational to eat, breath, have sex, love, make babies, have fun, and so on - so the rationalist will naturally conclude because his brain and being are coded to conclude as much. So be it. Humans aren't fully rational though, whatever the fuck rational may actually mean.
In any case, the idea that we can divorce ourselves from emotion is an illusion and is in many ways a potentially damaging self deception. Go without food long enough, or other of our primary biological needs, and the supposedly strict rationalist will very quickly go right off the rails. Pretend like you've got a "rational mind" divorced from your "emotional mind" and you're frankly drive yourself and others crazy.
On cultural relativism: I don't have a problem with judging how people acted in the past. There's a limit to my own tolerance regarding past behavior, and I believe everyone has a limit. My mentioning of Mohamed's 9 year old wife was to show one example. Maybe people did marry at 14 in the past more, but there's all sorts of things that used to happen which we now have concluded were incorrect actions. Incorrect now, and, incorrect then. For example: slavery, the oppression of women, having kings rule with an iron fist, and so on.
Pinker has spoken of an progressing moral zeitgeist as has Dawkins. Also Harris has some good ideas on a science of morality. Thank goodness we now have a much wider scope for our in group morality.
Down at BYU in the psychology & religion departments, they may strongly decry the "hedonistic" nature of "the world," and try to get their students to be wary. They go to their churches on Sunday where Mormon bishops interrogate youngsters about masturbation, and where all the youngsters learn to fear normal natural human sexuality. But, meanwhile, their founding prophets have been reasonably documented as doing things which the Mormon Church would excommunicate people for. So that's the point. Hypocrisy.
Don't masturbate Johnny, but meanwhile worship a god who had literal sex with the wife of another man. Don't have oral sex, Julie and Jim, but meanwhile Brigham & Joseph got to have sex with the wives of other men and with under age girls.
My own experience with Mormonism has made me more of a strong naturalist, on both sides of the cultural spectrum. So for example I'm all for people having as many babies as they want - if that's what deep down they feel like doing. The exuberance of the gays has a companionship with the exuberance of a large family. I try not to disparage either course of action because deep down people are doing what they feel included to do as animals. So be it. And, as rather intelligent animals maybe in the long term we can engineer and way to stay alive when the sun gets 10% hotter.
http://jonathanshome.blogspot.com/2010/12/funeral-talk-that-i-gave-in-february.html
The bottom line is that I don't think everything is relative. All societies past & present can for example be evaluated against the following yardstick: How well did each help humans be happy & thrive, and how much did each hold people down & make them unhappy?
http://en.wikipedia.org/wiki/Science_of_morality