A principle by which the universe operates, and which is fundamental to life and existence, is the principle of complementarity. By this I mean that all phenomena depend upon the interplay of opposites — things that are dissimilar to each other, and therefore work together in a complementary manner. Without this dissimilarity, the phenomena (whichever ones we care to discuss) simply cannot do what they’re supposed to do, maintain themselves, or even exist to begin with.
To state the obvious, existence itself is a complement to nonexistence. If we say something exists, that’s because that “something” cancels out its own nonexistence. This may appear a simple truism, but it’s a profound philosophical truth with extensive ramifications. Light, for example, is meaningful only in apposition to darkness, i.e., the non-existence of light. Matter and energy (really “two sides of the same coin”) are effective only to the extent that their presence contrasts with their non-presence. Otherwise matter wouldn’t matter — because it wouldn’t be.
The principle of complementarity extends to the field of information. As Gregory Bateson pointed out, information is “a difference that makes a difference.” That is, information is found in the difference between one state and what is not that state. A blank sheet of paper holds no information except in how it differs from its background, i.e., the information is found only at the edges. For a sheet of paper to contain information it must have some kind of markings on it which differ from the paper medium itself. There is no information in undifferentiated sameness.
Therefore, digital information also depends on the principle of complementarity. In a digital computer, a “byte” must be either turned on or turned off; there’s no half on or half off. The information in the DNA of living cells depends on the sequential ordering of the nucleotide bases along the spine of the molecule; each of the base pairs is either in one position along the sequence or another, and the positioning governs the information that is replicated into the rest of the nucleus to build the many types of proteins that enable the organism to function. The base pairs themselves (adenine-thymine, guanine-cytosine) are complementary; for example, a cytosine-cytosine combination doesn’t work because hydrogen bonding can’t occur between two such molecules.
Proteins themselves follow the principle of complementarity by folding into a three-dimensional shape that interlocks with whichever chemical they are designed to process. Unless the shapes of the protein molecule and the target molecule are complementary (that is, fitting like a hand into a glove), the two molecules cannot “nest” and the protein cannot do its work.
Mechanical and other objects also demonstrate the principle of complementarity. Take fasteners, for instance. Two bolts can’t be bolted together; to do what it’s designed to do, a bolt requires a complementary nut with threads of the same pitch and diameter. Or try fastening your jacket if both its edges have only holes, or only buttons. If your car battery had two negative poles, or two positive poles, you would never be able to get the starter to turn over. Two North American railroad cars can couple because the coupler knuckles face each other in opposite directions (always being right-handed as viewed from each car, therefore interlocking when viewed as a pair).
Now let’s apply the principle of complementarity to biological life. Both male and female parents are required to reproduce offspring. Without a complementary union of gametes (ovum and sperm), fertilization and a resultant zygote (the beginning of a new organism) doesn’t occur. An ovum can’t fertilize itself.
The principle of complementarity makes it clear why homosexuality is a ludicrous concept and a practice devoid of function — like trying to start your car with two negative poles on the battery. The requisite pairing of complementary body parts is absent from intimacy between two individuals of the same sex. There’s a logical reason, grounded in the structure of the universe, why marriage is appropriate only for the complementary pairing of male and female.