Even if a post is a bad post for some reason or other, it is probably not offensive. The Offensive flag is meant to be used only in extreme cases, like hate speech, or abuse.

For example, if a user posts obscene images to the site, that should be flagged as offensive. But if someone says something bad about your favorite technology, that probably doesn't apply.

As a rule of thumb, if you can't justify something as being hate speech, or abuse, you shouldn't mark the post as offensive. Instead, you should down-vote the post.

When you decide to flag a post Offensive, you will get a warning dialog. Take this time to decide if the post is really offensive.