A non Jew who is intellectually honest understands that the normal marriage relationship produces children, and the other types of marriages do not. He understands that historically all religions have considered this type of behavior immoral; that in the past doctors and psychologists considered it deviant. He can see that ONLY in this generation of “all is ok” have the natural order of things been twisted and changed.
He might say – why are so many people in the non Jewish world not against this lifestyle? The same way the Nazi soldiers chose the path of evil and cruelty despite their natural human kindness G-d created them with, people can choose to pursue a corrupt, deviant way of life to such an extent that they no longer OBJECTIVELY can recognize the NATURAL way the world was created to function.