[Hebrews 11:6]

You and I agree that scripture teaches homosexuality’s sinfulness as a chosen behavior, and because we accept scriptural authority that settles it for us. For those who do not accept scripture as the final authority, other arguments are needed.

[Hulka]

To me it is this:
1) If you believe in the Bible, then man and woman were created to perform two different sexual functions/roles.
2) If you believe in evolution, then man and woman evolved over millions of years to perform two separate and different sexual functions.

Therefore, to engage in homosexual behavior is to act against God or nature.

Last thought: If, say, they discover some sort of genetic component that “makes” someone homosexual, that means being born that way is the result of a natural process. However, it is not normal.

Much like being born with a bi-cuspid valve in your heart instead of a tri-cuspid valve, this condition occurred naturally but is not normal. Same with homosexuality

Just my two cents. . .