I don't think it's swearing as much as offensive content. If you base this restriction on the assumption that a sizeable portion of the player-base is underage (or at least near the start of puberty), then throwing sexual vocabulary at them is, indeed, wrong.
This, of course, limits a great deal of people's scope of swearing, since a lot of the terms we use to swear pertain to sex. Some words, though, like crap or shit, I imagine, are fine, unless of course you use them to 'offend', 'abuse' or 'harass' someone.