The teaching that "all sins are equal" is another one of those garbage doctrines that defies both scripture and common sense, yet are popular among Christians.
The only truth there is to it is that even the smallest sin separates us from God. Just the same, conviction for even the smallest crime leaves you with a criminal record.
Why do people believe such garbage? I think the biggest reason is because cowardly Christians are afraid of being hated by the world. "No, no, no, I'm not any better than you. Sure, you rape boys and married another man, but I'm the chief of sinners!"
Yet it's shockingly prevalent in the Church. It's a way to avoid dealing with serious sins.
Takes me back to Paul's admonition of a serious sin in the Corinthian church.
1Cor. 5:1 It is actually reported that there is sexual immorality among you, and such sexual immorality as is not even named among the Gentiles—that a man has his father’s wife! 2 And you are puffed up, and have not rather mourned, that he who has done this deed might be taken away from among you. 3 For I indeed, as absent in body but present in spirit, have already judged (as though I were present) him who has so done this deed. 4 In the name of our Lord Jesus Christ, when you are gathered together, along with my spirit, with the power of our Lord Jesus Christ, 5 deliver such a one to Satan for the destruction of the flesh, that his spirit may be saved in the day of the Lord Jesus.
I could just imagine how modern churches would have dealt with this. "Yes, but we're all sinners, and this is just one of many sins. We're about love and forgiveness, not judgment, yada yada yada."