Is profanity sinful/morally wrong?

[Swing low, sweet chariot. Comin' for to carry me home.]

why do we have to have these discussions?

Luke 6:5 For out of the overflow of his heart his mouth speaks.

Eph 5:3-4 3 But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for the Lord's people. 4 Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving.

[Swing low, sweet chariot. Comin' for to carry me home.]

Kevin Smith is a Christian. Take a look at his movies.

oh, I see, the actions of my fellow Christians are my standard, not scripture?!

look, I defend what is written in scripture; not the arguments of men

[Swing low, sweet chariot. Comin' for to carry me home.]

Does it harm anyone against their consent?

Yeah, that's what I thought. No.

that's the defintion of legality, not morality.  

The law doesn't, nor should it, make immorality illegal.  Otherwise, there would be no religious freedom.

What the law attempts to do, and what it should do, is defend the life and property of others.

[Swing low, sweet chariot. Comin' for to carry me home.]

that's the defintion of legality, not morality. 

Nah, not even the definition of legality.  It's the definition of morality to many of us secular folk.  The law is perfectly happy to sit idly by while you smoke and drink yourself to death with independent children.  On the same token, it's happy to allow parents to do virtually anything short of beat their children to death without legal intervention.  That ain't legality.

in your case, beatings are understandable

[Swing low, sweet chariot. Comin' for to carry me home.]

In the same breath, our culture has deemed these words offensive for whatever reason (or lack thereof, more often than that). With that in mind, it's probably best to abstain in order not to cause a fellow believer to stumble or confuse the mind of a non-believer into thinking you are being a hypocrite if they don't understand the nuances. See Romans 14, which, while not specifically addressing the use of culturally offensive words (such an idea did not exist when scripture was written), contains tenets that I think aptly apply.

I agree Romans 14 commands us not to offend others over non-salvational issues.  The commands of Romans 14 are situational and apply only to situations where you are in the company of those who may take offense.

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What we arrive at is, not surprisingly, that their usage as sinfully or morally wrong (from a Christian perspective) is a grey area. In mixed company, however, it is better to abstain for their sake.

fine, but that doesn't mean profanity is acceptable when NOT in mixed company.  Example:  it wouldn't be morally acceptable for a group equally yoked believers to curse up a storm while golfing.

The following passage is NOT situational:

Eph 5:3-4 But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people. 4Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving.

[Swing low, sweet chariot. Comin' for to carry me home.]

I have to wonder what the Biblical definition of obscenity was.  I doubt saying "F**k you!" counts as an obscenity in the eyes of G-d.

Matthew 5:37
Simply let your 'Yes' be 'Yes,' and your 'No,' 'No'; anything beyond this comes from the evil one.

James 5:12  Above all, my brothers, do not swear—not by heaven or by earth or by anything else. Let your "Yes" be yes, and your "No," no, or you will be condemned.