I like the intentions of this post, however I think the reasoning behind your argument is weak.
Overall presenting that situation is just representing a circular argument, since you are pretty much telling that person we have decided to rate the servers PG-13, even though that begs their argument that the in-game characters swear, or similarly "why not turn on the language filter [if you are afraid of children being exposed to language]?".
In my opinion, a better response to the "but the game says vulgar words" would be this:
Hearing a fictional character spew "bad" words in exclamation for the intention of furthering a sense of excitement in the game is vastly different than knowing that a real person your playing with is purposely insulting you with the sole intention of making you feel bad.