The Hebrew word for 'iniquity' is 'avon' (not pronounced like the makeup, but ah VONE). It means to twist or to bend.
The iniquity in man means that he is a bit twisted in his makeup. The interaction between G-d and man has a twofold purpose:
Note, in the above, man/men are generic terms for male and female. There is no intention to elevate nor denegrate women.
Shalom.
It is not human nature to sin. The tendency to sin is the result of the absence of grace at conception, and the consequent damage to human nature brought about by the absence of grace. "Human nature" considered as "what is usual" or "what is common" obviously includes the tendency to sin. But "human nature" considered as a source of values, is what God has created, properly developed and properly ordered to the proper ends of the human person and his many faculties. Since homosexual desire is the desire to do something that is destructive, it is obviously contrary to the good of human nature.