They'll get over it. And anyway, tis better than being eaten by a dog or getting fleas or heartworm or being run over by a car. All three of our cats are indoor kitties (because the cat we had before them was squished by a car right in front of our house SNIFF). And the funny thing, they were all born on the street, abandoned at a young age, etc. But, they've learned to deal with staying inside. Plus, open windows (with screens, of course) are their friends. It's a big thing when I open a window for 'em, because it's like, "YAY! Disneyland!"

But, yeah, they'll get over it if you lock 'em in. And in the long run, they may even learn to love you for it.