[Mild language warning.]
Go to your silverware drawer and pull out a metal utensil—a fork, for example—then show it to the next person you see and say, "It's just a piece of metal."
Aside from the confusion of having someone randomly show them a fork and declare the obvious, most people would disagree with the "just" part of the declaration because even though it is true that the fork is a piece of metal, it is clearly not true that it is just a piece of metal; it is a piece of metal shaped into a tool for a specific purpose.
Now say "fuck" to the next person you meet. When it makes them uncomfortable tell them, "It's just a word."
Like the fork, words are tools—tools designed to communicate, to call forth meanings from within another person's understanding. When someone uses words in contexts that many or most people consider offensive and then tries to text-justify it by saying, "it's just a word," they are in essence trying to modify the purpose of that word, or worse.
It is a choice to use words we know others are likely to find offensive, especially in contexts where that are clearly merely cursing or trying to be offensive, but we have to acknowledge the purpose of our choice rather than try to deceive ourselves and those we are interacting with.
I am not suggesting that there are words that are exclusively offensive, because—as I noted—context plays a role in communication. I am saying that the phrase, "it's just a word," is not a sensible or valid justification.