Watching television in itself is not necessarily bad for you, unless you do it too much. It's the sitting that is bad for you; new research is showing more and more that sitting for long periods of time will really hurt your health. This recent article in the New York Times explains some of the dangers: http://www.nytimes.com/2011/04/17/magazine/mag-17sitting-t.html?hpw
If you want to watch television for a bit, try doing some exercise while you watch: use an exercise bike, treadmill or balance board, or use some light weights. If you can't do any of those, even standing while watching will be a bit better for you. Good luck!
Watching TV can be bad for your health physically because people spend more time sitting and they do not exercise. So if people cut down on watching TV and exercise or better yet, exercise while watching TV most people will not be overweight. Also, TV can be bad for your health mentally if you are constantly watching sad depressing programs including the news. To change that, watch a funny comedy special.
A small amount of TV can be ok for you because it reduces stress and helps you relax. On the other hand too much of it (as with anything) is bad because it'll cause you to become lazy and also watching TV puts your mind in an extremely lazy state. Your mind is more active asleep than it is watching TV so over time you'll lose cognitive power. Most Americans are dumb and slow witted because they watch TV constantly. On the other hand if you read books and challenge your mind frequently you'll get more out of life.