The Monty Python segment above is very amusing but it actually is based on medieval philosophy as practiced by 12th century thinkers like Thomas Aquinas. It also makes a salient point. Your logic is only good if your starting assumptions are correct. Thomas Aquinas was a genius with logic skills of the highest order and is largely considered the father of Catholic Natural Law. There are many schools of natural law, but the one most frequently cited in the gay marriage debate is the Catholic variety so I will stick to that. Aquinas main theological project, The Summa Theologica, is foundational to natural law and was concerned with reconciling Aristotle’s philosophy with Christian theology. Galileo got into all that trouble with the church roughly four centuries later by upending Aristotle’s cosmology which, thanks to Thomas, was considered settled theological truth. Aristotle’s metaphysics included the notion that all matter is formed of four essences, water, fire, earth, and air. Therefore birds could fly(or ducks float!) because of a certain admixture of air in their physical composition giving them a tendency to rise. That's only slightly less absurd than a wooden duck but it made sense at the time. In addition he reasoned that living things also contained a spirit or life force that directed the actions of living beings. This explained the difference between the living and non-living. In addition human beings had another non-physical component called a genius which corresponds to higher reasoning. This is what separated man from the rest of the animal kingdom. Aquinas took these concepts and placed them in a Christian frame. Without going too much further down the philosophy rabbit hole suffice it to say that Aquinas thought the human intellect had a divinely built in capacity to recognize moral choices. There where two sources of moral knowledge, natural laws and divine revelation. Natural laws are self evident in nature, like the will to live, the desire for food, or the desire to procreate. That last one of course is the sticking point in the gay marriage debate. Divine revelation was generally derived from scripture. In Thomas’s system all natural behavior in animals was intrinsically good. Only human beings, through the exercise of free will, could subvert their naturally moral state. Thomas states...
“It must be said that every evil in some way has a cause. For evil is the absence of the good, which is natural and due to a thing. But that anything fail from its natural and due disposition can come only from some cause drawing it out of its proper disposition.”
Basically since animals lack free will there is no cause to make them commit an evil act. Of course there is a disconnect here with modern science. Other species in fact due posses various levels of cognition that lead to varying degrees of what could be described as free will. Chimpanzees not only commit murder but they group together and plan ambushes ahead of time. A chillingly human act of evil. Another disconnect, in Aquinas’s natural law the only purpose of mating is procreation. In his time no one seems to have been aware of any counter examples from nature. Of course we know now that animals enter into same sex bonds for companionship. In fact same sex pairing, complete with sexual behavior, has been observed in over a thousand species. And don’t forget the Bonobos, those swinging free love hippies of the animal kingdom. Aquinas was operating on false assumptions. Basically what I’m saying is even if your logic is sound(Aquinas was impeccable), you make erroneous judgements about truth if, like the characters in the Python piece, your underlying assumptions are wrong. This means that appeals to natural law to oppose gay marriage just don't hold water. In fact with proper updating a natural law argument could be made for gay marriage. It won’t happen though. No matter how many gay penguins you roll out the Catholics won’t budge. Thomas Aquinas left them a trump card. Remember that divine revelation bit?
”But as to the other, i.e., the secondary precepts, the natural law can be blotted out from the human heart, either by evil persuasions, just as in speculative matters errors occur in respect of necessary conclusions; or by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle states (Rm. i), were not esteemed sinful."(emphasis mine)
Natural law provides an unimpeachable guide to moral behavior unless the Apostle Paul say’s it doesn’t. A witch! A witch! No wonder you can’t reason with these people.