No matter how we interpret the word “natural”, this claim is false.(1) That homosexual activity is observed in nature among other mammals is incontrovertible. Even if no other mammal engaged in any homosexual activity, there is no question that such activity occurs among humans, meaning that it is indeed entirely “natural”.(2)
If the claim is that homosexual activity is “unnatural” because it does not lead to procreation, then we would have to condemn all sex that is not aimed at procreation as equally “unnatural”, and may have to consider preventing married adults who are not parents from being ordained - not to mention anyone on birth control, anyone who masturbates, etc.(3)
If the claim is that homosexual activity is “unnatural” because it is dangerous or perverse, we should bear in mind that there is nothing activity-wise that LGBTQ persons do that straight persons do not do in far greater numbers.(4) Homosexuality is natural by any reasonable definition of the word.
1. (For the issue of "complementarity", see below) Let's set aside for a moment the fact that almost nothing the Bible enjoins human beings to do is strictly "natural", and that calling something "unnatural" isn't really a significant argument against it. In many cases, "homosexuality is unnatural" seems to be code-language for "I find homosexuality disagreeable" - where it is not simply a result of a poor working definition of "natural".
2. The most common use of the word "natural" is to mean "occurring regularly in nature", or "in accordance with natural principles". Homosexual behavior has been widely documented in thousands of animal species, including examples of monogamous life-long pairings. It occurs regularly in nature and is in accordance with natural principles.
3. Some people argue that the only 'valid' sex acts are ones that might result in procreation - and this is often the implicit argument when someone is talking about what is "unnatural" with regard to homosexual sex acts. Not only does this line of argument ignore the obvious social, pleasure, health and psychological dimensions of sexual behavior it is used in mysoginistic ways detrimental to public health by condemning contraceptives and prophylactics. This idea is often supported by the clumsy argument from supposed design - object A fits into object B, and therefore, that is the only acceptable configuration. We hope you can see why this is absurd, because stuff like this is beyond parody.
4. Regardless of what sexual activity one is considering - anal sex, oral sex, masturbation, you name it - heterosexual individuals and couples are engaging in it in far greater numbers than LGBTQ persons. The recent National Survey of Sexual Health and Behavior reveals, as all past studies of this type have, that human sexual behavior is highly varied and that practices often treated as taboo in our public discourse are in reality widespread. Of course, any sexual behavior carries risks and every one is responsible to take care of their health and the health of their sexual partners, but it is not a unique homosexual issue.
Complementarity, or Complementarianism
Complementarity, or complementarianism, is a core argument often put forward under the rubric of homosexuality as unnatural. In brief, complementarianism is a Christian theology which states that men and women have very specific, God-ordained roles to play in life and society. This idea is nothing new - we can go back to Aristotle and further to find the concept that men and women should both stick to their ordained places. For men, unsurprisingly, this God-ordained function is wielding authority - in contrast to women. If you are scratching your head and thinking "complementarianism is just patriarchy dressed up in drag", you are correct. It is an old argument, that women should remain 'in their place' because they are designed to be subservient child-bearers only, but not an argument we owe any attention.
Certainly, people in the ancient world believed in fundamental differences between the sexes which under-girded their misogynistic societies. Some of these beliefs wormed their way into the Bible as well. Fortunately, the power of the Holy Spirit is present throughout scripture, breaking down barriers between people, undermining arguments against equality, in Paul's majestic restatement of a baptismal creed in Galatians; in the many stories of strong women in the Old Testament including Esther, Deborah, Miriam, Ruth & Naomi; in the priestly expression of the imago dei; in Elizabeth's faith; in Mary's courage who became the only human to give birth to God; in the constancy of the women at the cross and the tomb; and Mary Magdalene's commission as the first evangelist.
Those arguing for complementarianism ordinarily base their argument on the pairing of male and female in the creation narratives, then repeated various times in the Old Testament and the New as the basis of God's design for human relationships. They choose to privilege this theme in scripture over others which affirm a wide diversity of human relationships. Most significantly they overlook Jesus' frontal assault on the concept of family, and Paul's largely negative view of marriage in contrast with celibacy.
Setting the Bible aside, the design of male and female genitals is taken as evidence of complementarianism. Really, however, this is basing a complex argument on a basic observation that simply does not sustain it. It takes little imagination to come up with more than one configuration for sexual activity. Arguing from human design to support inequality is something western society is fighting hard to end permanently. The church should not be propping the door open to allow in poor logic which has been invoked to deny rights to women.
Apart from issues of equality between men and women, complementarianism also fails to justify denying rights to LGBTQ persons for all the reasons it fails to justify denying rights to women.