Of course, IcftB has its rules posted in the guild info, so everyone knows how to behave and what to expect from others. Posting rules works well that way. But if no chat guidelines are posted, people can still tell what's appropriate by just seeing what the veterans do. If the topics and language tend toward an R rating and someone gets upset by profanity, that someone is in the wrong guild.
A couple of years ago, we covered a related topic in Drama Mamas when a parent freaked out because her tween heard some rather raunchy language in guild chat. It seemed that late on most Saturday nights (and on into the early mornings), many guildies came home inebriated and a bit more foul-mouthed than usual. While this was a regular occurrence, the mother claimed that her 12-year-old was on and overheard it. Was she in a greatly different time zone, or did she really let her child stay up that late to play video games? We don't know. But if guild chat gets raunchy at the same time every week, sensitive ears should probably turn it off during that period.
I'm in another guild made up of mostly guys, and the standard tone of chat is high school locker room. When they get really gory or offensive, I just make my presence known, and it usually rolls back to an R rating without drama. That's the difference between immature players being profane because they can and mature people participating in fun but raunchy conversations -- the mature players know what's appropriate and adjust accordingly.
What are the chat rules in your guild? Are they posted or just understood? Do they vary according to time? And what, if anything, happens when the rules are broken?