Human sexuality may have some mechanical similarities to animal sexual reproduction, but remember, any act which involves the human free will makes it a human act and not just an act of man (a biological function).
All human acts are moral acts. A man or woman must freely choose to have sex, and that act of the will makes this a moral act. When human sex is used properly, it is a holy and sacred sign of the covenant between husband and wife. When done to promote love (unity) and life (procreation), it is a marital act and is sacred in the eyes of God.
When unmarried people have sex, before or outside of marriage, it is the sin of fornication. When one or both persons is married to someone else, then it is the sin of adultery. When done by yourself, it is the sin of masturbation. When it involves explicit media (Internet, telephone, television, photos, videos, or text), it is the sin of pornography. When done with persons of the same gender, it is considered the sin of homosexuality. (While the sexual inclination and orientation may be inculpable, all sexual activity between individuals of the same gender is considered immoral, just as all sexual activity of unmarried heterosexuals is considered immoral and sinful.)
Any and all sexual activity outside of marriage (that is, the marriage of one man and one woman in a permanent, faithful, and fruitful union) is considered immoral and sinful. Modern culture portrays sex as a human right. The Bible and the Church say otherwise. Sex is a gift from God; what distinguishes human sex from animal sex is that human beings act morally—with their free will and not out of blind instinct. People choose to have sex, whereas animals just do it when they are so inclined.
Human sexuality is reserved for marriage alone. We reserve the consumption of alcohol for persons over the age of twenty-one. It is illegal to buy or sell dangerous drugs such as heroin, cocaine, or other controlled substances. None of these is a denial of human rights or freedom. Even married couples cannot perform the conjugal act in a public place, and no one considers that unreasonable or unconstitutional.
The Church does not see human sex as intrinsically evil or sinful. The heretical sects, like the Gnostics and Manicheans, were that fanatical. The Church merely reiterates what the Natural and Divine Laws already make clear: sex is reserved for husband and wife. Even then, spouses should not be seen or used as mere sex objects or sex toys; rather, married men and women ought to see their spouses as their beloved to whom they are covenanted for life.