Is it animal abuse to keep an animal(cats) indoors only?

I keep all 3 of my cats indoors. They seem ok with it. As long as when it's really nice out you open a window, put in a screen and let your feline catch some of the breeze, I think it's ok. Also make sure during the day you keep your shades pulled up so he/she can "sun bathe". Also, strictly indoor cats tend to be more domesticated, meaning friendlier and less aggressive.