...Natural Law
People have a basic, ethical intuition that certain behaviors are wrong because they are unnatural. We perceive intuitively that the natural sex partner of a human is another human, not an animal.
The same reasoning applies to the case of homosexual behavior. The natural sex partner for a man is a woman, and the natural sex partner for a woman is a man. Thus, people have the corresponding intuition concerning homosexuality that they do about bestialitythat it is wrong because it is unnatural.
Natural law reasoning is the basis for almost all standard moral intuitions. For example, it is
the dignity and value that each human being naturally possesses that makes the needless destruction of human life or infliction of physical and emotional pain immoral. This gives rise to a host of specific moral principles, such as the unacceptability of murder, kidnapping, mutilation, physical and emotional abuse, and so forth.
Homosexuality
Homosexual behaviour is wrong using the same Natural Law reasoning that murder and torture is wrong. It's wrong because it is unnatural. That is the truth and, obviously, the truth is not always agreeable.