If the cat has been inside from being a kitten the answer is no it isn't against nature. I was told by a co worker that it was cruel to keep them indoors, but as my vet pointed out if their kept in from being a kitten then they don't know any different.
Its nature is to run, jump, pounce, catch prey, sleep, lie in a sunny spots etc. as opposed to being outside per se. Those needs can be met indoors. I do think it's good if a cat can go outside into an enclosed garden or enclosure, or with a harness but I don't think they suffer by not being able to. Especially if they've never been outside.
I second this! My Polly is 16 months old. She loves to look out the window and lay in the sun right next to the door, leaning against it. But if you open the door and hold it open, she won't try to run outside or anything. I don't think she understands that there is an outside. It's like she thinks it's just a big TV.
