I've been reading some heated arguments over homosexuality recently, and I am struck by the sheer quantity of fallacious arguments used by the homophobic contingent. Saying "If homosexuals were okay then they'd be able to breed" is like saying "If we aren't supposed to eat humans, why are they made of meat?" or "If God meant for us to walk around naked, we'd be born that way."
The preceding claims are examples of the Appeal to Nature and teleological argument fallacies. (Related: Appeal to complexity, appeal to design, cosmological argument/anthropic principle, appeal to norm.) The appeal to nature is often used to sell herbs and supplements: "They're natural, therefore they are good for you!" Well, both vitamin C and uranium can be found in nature, as can both poison ivy and apple trees. The teleological argument primarily deals with the existence of God, but goes beyond that to claim that since certain structures and systems work a certain way, that must be the right way (since, according to the argument, it was all created by some supreme being.) I don't think I'll even address this fallacy here, since the Wikipedia article I linked to contains such a fantastic analysis.
Ultimately, this comes down to a confusion between science and religion, between logic and faith.
Science is founded on observation
Science, on the one hand, combines and analyzes observable evidence to posit patterns, then experiments with those patterns in a range of situations to test their validity. These patterns, known as "theories", can then be used tentatively generalize to unknown regions of knowledge, until those regions can be more thoroughly explored. When a theory is sufficiently observable and/or well-established and matches the evidence, it becomes known as a law. The process of science is logic, and the only axiom is that the truest picture of our universe can be gained through observation. Gravitation is an observable law. Evolution, contrary to media terminology, is both an observable and well-established law, though the mechanisms and intricacies are still at the level of theory. Global warming is not a theory, it is purely an observation -- the speculations as to its cause are the theories.
Religion has arbitrary axioms
Religion, on the other hand, starts with a large set of axioms -- pre-existing beliefs that tend not to change over the short term, even in the face of contrary evidence. (This includes gut-instinct ethics, which is in turn influenced over time by contemplation and observation.) From this set of axioms is derived (also through logic) a set of other beliefs. This process of derivation differs from that of science in that science is built only on data and the Observation Axiom, whereas religion incorporates relatively inflexible axioms.
On the third hand
Real life involves occasional conflict between these two approaches. Unfortunately, the very process by which the conflict is resolved is often delegated to the faith approach! Some prefer to allow religion to conquer all, sometimes leading to embarrassing inconsistencies, while others have religion fill in the gaps where science cannot reach, if only for the moment -- and it does that very well. (You can tell which side I'm on.) When humans are faced with a dilemma and insufficient data, they have several sources of insight: Evidence-based logic (science) and faith-based logic (religion). The faith-based logic is less limited in scope, since it can readily incorporate new axioms at a whim, allowing harder-to-arrive-at results. The evidence-based logic is of course more restricted, but also more foundationally solid.
This is where the trouble starts
In a debate, a person might take a faith-derived belief and treat it as an evidence-derived belief, leading to logical fallacy. For example, the appeal to nature is a logical fallacy only because it is taking for granted the statement that "everything good is natural". Within a religious context, this is not a fallacy. Same with teleology: If 1) everything was created by some God, 2) everything has a purpose, 3) we have the ability to determine that purpose, 4) straying from that purpose is going against God, and 5) going against God is bad, then Conclusion: anything that does not do what it usually does is bad. (That' a lot of axioms right there, folks, and #3 is especially suspect.)
What of public policy?
Since science has shown such amazing coherence and agreement across cultural divides (due to the Observational Axiom) and religion has not, it only makes sense that science be the foundation of public policy. There are always edge cases, of course, places where science shrugs and leaves the burden on ethics. That's all good, but we're not even there yet -- my own goverment (U.S. of A.) still actively refuses to look at some of the hard data on climate, pollution, biodiversity, and health, simply because the data conflicts with some of the arbitrary axiomatic articles of faith the policy leaders espouse. Such beliefs are best left in the realm of faith and religion, where they properly belong, not in the realm of science and logic. Each has its own validity -- but public policy should perhaps be mostly influenced by the latter, not the former, as long as basic ethics are also honored.