Your title for this thread is Scripture and Homosexuality, well, Scripture says sex between men (**ARSENOKOITAI) **is sinful. So, using the Bible, it is sinful because the Bible says so. This means that homosexual sex violates Divine Law. But, sexual acts between two men or two women is also sinful because it violates the Natural Law.
Natural Law asserts that 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 bestiality—that it is wrong because it is unnatural.
We know from reason that a man’s reproductive organ was not intended to be inserted into another man’s organs intended for waste evacuation. It is obviously* unnatural *to do so. This is why homosexual acts violate, also, the Natural Law.
catholic.com/library/homosexuality.asp