Paul is addressing this on a moral level, not a physiological level. This interpretation introduces a whole new concept that is entirely foreign to the text. This argument, therefore, self-destructs. The body was built to function a specific way. Men were not built to function sexually with men.
Natural desires go with natural functions.
The passion that exchanges the natural function of sex for the unnatural function is what Paul calls a degrading passion. The desire is unnatural because it abandons the natural function. Extramarital heterosexual sex is wrong because it exploits a natural function in an immoral way. Homosexuality is worse because it's an immoral act based on the perversion of a natural function. Reproduction permitted for non-commercial use only.