“Why is it that we Christians spend so much time and energy objecting to the first sin. . .”
I think it’s because the homosexual groups are militant, and forcing social acceptance through our schools, military, and laws.
The other “sins” you mentioned (I am not sure myself about all of them, but whatever) are not being promoted with flags on the street, newspaper editorials, big fat parades, etc.
And you aren't threatened with criminal action (threat of deadly force from the government) for speaking out against contraception and divorce.