Penises were made to go into vaginas therefore any other use of penises or vaginas is a violation of their natural purpose and a sin. So goes one argument for the sinfulness of homosexuality. This line of argumentation is an appeal to Natural Law.
Natural Law is another form of deontological ethics. Natural Law assumes that true, abiding moral principles are immanent in nature. Natural Law is discovered, not created, thus it is purer than human laws. In Christian ethics Natural Law is assumed to participate in and accord with the Eternal Law which is God's intentions for creation. Natural Law is often supplemented with (but never contradicted by) Divine Law, which is the revealed set of laws given in scripture and by the Church.
Being a form of deontological ethics it has the same limitations as Divine Command ethics I described earlier. It necessarily restricts itself to universal principles which are extremely rare. It cannot say much about specific human behaviors, because rules which govern human behavior nearly always require exceptions. It has no room for consideration of consequences, intentions, or circumstances. It is a blunt instrument.
The same Natural Law argument which would declare homosexuality a sin, for example, must, to be consistent, also label as sin any form of sexual behavior which does not serve the supposed final cause of sex, which is reproduction. That means that oral sex, anal sex, masturbation, intercourse during menstruation or after menopause, or when either of the parties are known to be infertile, or when any form of contraception is used must also be ruled out. To temper the obvious ridiculousness of this standard apologists will say that the sex act must only be "open" to reproduction, which they interpret to mean vaginal sex without use of contraception. They will say that sex with a post-menopausal woman, for example, is still "open" to procreation based on the example of Sara and other Biblical stories where God works a miracle in a woman of advanced age. But if we're including miracles in our rationale then any kind of sex must be "open" to procreation because nothing is impossible with God. In fact, I can think of one notable example in which there was no sex at all and a pregnancy still resulted.
The point is, to be consistent with this line of argument you must treat a wide array of sexual behaviors as equally condemnable with homosexuality. Masturbation is just as "unnatural" by this thinking - yet it is essentially universal.
Indeed, the irony is that saying it is "Natural Law" that penises should only be inserted in vaginas (and only for the purpose of procreation) goes against one of the foundational tenets of Natural Law theory - which is that true laws are immanent in nature, and thus deducible by observation. It is easily observed that there are actually a variety of uses for penises and vaginas aside from procreation, just as we use our mouths to breathe, to eat, to talk, to sing, to smile, to kiss, to whistle, to spit, to cough, to vomit and more. If Natural Law can be deduced by observation it ought to be susceptible to the evidence of the Biological sciences (more on that in a later post), which clearly show us that masturbation is natural and purposeful, and increasingly show us the same about homosexuality.
Natural Law is such a rude and useless instrument that not only is it limited to dealing with a few supposedly universal principles, not only does it often fail to accurately teach us what is natural, but it also completely fails to adequately demonstrate the connection between what is natural and what is good. By this reading of Natural Law there is nothing to distinguish rape from consensual heterosexual marital sex for procreation. As long as "tab A" goes in "slot B" the sex is "natural", but how can it in any sense be called good? To begin to approach what makes sex good or bad we have to examine consequences, circumstances and intentions - all things which a deontological method ignores.
Conservative moral reasoning swims almost exclusively in these deontological circles. Homosexuality is sinful either because "the Bible says," or "God commands" or because "it is unnatural." It is the persistent failure of these approaches that they focus exclusively on a narrowly understood sexual act, reducing all of human sexuality to the moment of coitus. They can have nothing to say about the morality of hand-holding, kissing, flirting, foreplay, or the deep emotional bonds formed in sexual relationships. They fail to address people in their sexual being, and thus they're found by most of society to be irrelevant. There is a reason why the Roman Catholic Church's teachings on sexuality are almost totally ignored.
We can do better.