Basically, William Apess finds the laws against intermarriage upsetting because it highlights the hypocrisy of those who say they believe in the Scriptures but behave contrary to its beliefs. Apess finds it disgraceful that a fifty pound fine has been levied against any clergyman who unites whites and Indians in matrimony.
Apess also believes that the right to choose a marriage partner belongs to the individual doing the choosing. After all, he states the case that whites quite willingly choose partners for life among the Indians; he questions why Indians may not do the same.
...for many have broken the ill-fated laws made by man to hedge up the laws of God and nature. I would ask if they who have made the law have not broken it...
...as the whites have taken the liberty to choose my brethren, the Indians, hundreds and thousands of them as partners in life, I believe the Indians have as much right to choose their partners amongst the whites if they wish.
William Apess argues that the white man's principles are only skin deep.
Hope this helps!