Homosexual acts by their very nature contradict the human nature we have been endowed with by God. They are a contradiction of who we are and have been designed to be by God as male and female. The Bible expounds on this Truth and gives us deeper insight to how God has made us and why, but you don’t need the Bible to be able to tell homosexual acts are a perversion of how we were designed to work. What really is being argued is whether this perversion is a bad thing or not.
Understanding why its wrong requires an understanding of what God designed sex for and what marriage really means. This concept is the core of what JPII talks about in his book “The Theology of the Body”. When you understand what sex is all about and the purpose God intended for it, homosexual acts seem absurd. This understanding can come about just through a study of the body without considering divine revelation, but if you have divine revelation to help you get there why not use it?
The Declaration of Independence makes it clear that the founders of our country recognized and understood that humans have a specific human nature endowed by God that should be recognized and upheld. They made this clear when they spoke about “inalienable rights” and “self-evident Truths”. All humans laws are subject to this natural law. While legal ambivalence is permissible on many issues, it is never permissible to have a law that directly contradicts natural law.