The You Don't Say? campaign comes from Duke University students. The idea behind the campaign is to encourage everyone to think before they speak, because sometimes what we see as being harmless can be offensive to someone else. In particular, those words that we use loosely that also have a very negative definition behind them.
Although most of us don't have bad intentions when we speak to people and use some of these words, we still are not being conscious of any negativity we may be conveying. Some phrases that are said with harmless intent are bitch, man up, no homo, that's so gay, don't be a pussy and many others.
The You Don't Say? campaign specifically addresses the misuse of language that often relates to the LGBTQ community.