According to the U.S. Federal Communications Commission (FCC), it's OK to say "God damn it" on TV, but it is not OK to say 4-letter words referring in slang to bodily processes. So society considers saying slang words for bodily processes to be more of a sin than explicitly violating the 3^rd Commandment.

 

To think that vulgarity is a sin is actually blasphemy, because it's putting society's rules above God's, and that's vanity, and that's the worst sin of all. So why do we take a stronger position against vulgarity than true sin?

 

Polite society considers it to be rude to talk about bodily processes. Polite society also values spirituality. Hence polite society is a combination of religious and social rules. Once they're mixed together, it's just a shade of gray to get them slightly re-prioritized. And as is typically the case, the sinners come to dominate, especially in social institutions. Before long, we see that it has become fashionable to think of God as our servant, but we wouldn't consider saying crude things in public. The Devil's work is done.