Arguing morality from nature is Dumb. Liberals do it, conservatives do it, Atheists and Evangelicals do it, and it remains dumb, dumb, dumb. Here's some of why:
Nature doesn't require enforcement. You can survey federal, state, and county law as long as you like without finding a statute forbidding that objects fall upward. Each day, the sun rises without fearing punishment should it fail to do so. Geese migrate and bees make honey, and there's no arrest or trial for the dissidents. What's truly natural takes care of itself--that's one reasonable definition of the natural.
The boundries of the natural are always drawn arbitrarily and self-servingly. What logical line divides condoms from umbrellas and tothbrushes in alienating people from 'the consequences of their actions?'
Religious conservatives have particularly odd views of what constitutes the natural. Many, of course consider homosexual sex to be unnatural. Orson Scott Card, a conservative Latter Day Saint, argues the opposite, that heterosexuality is unnatural:
Men and women, from childhood on, have very different biological and social imperatives. They are naturally disposed to different reproductive strategies; men are (on average) larger and stronger; the relative levels of various hormones, the difference in the rate of maturity, and many other factors make it far, far easier for women to get along with other women and men to get along with men.
Men, after all, know what men like far better than women do; women know how women think and feel far better than men do. But a man and a woman come together as strangers and their natural impulses remain at odds throughout their lives, requiring constant compromise, suppression of natural desires, and an unending effort to learn how to get through the intersexual swamp.
In his scheme, it's kind of amazing that the human race survived long enough for civilization to arise.
And don't think you're off the hook, liberals, with your gay penguins. By letting in the premise that homosexuality outside the human race is relevant to the discussion, you've made a hash of your logic.
Nothing is more natural than the strong preying on the week; nothing is more natural than the wasp laying her eggs in the caterpillar so that her young can devour him from within. Don't you think we can do a little better?
Now, while the natural is a terrible reason to forbid something, it is a very worthwhile consideration in how to go about prohibitions. It is natural for adults to lust after adolescents, and occasionally even vice versa. That certainly doesn't mean that acting on that lust should be legal, but it does mean that policy and morality that assumes that only monsters feel those urges is being set up to fail.