I stopped when a neighbor's dog attacked my girl and brain-damaged her so badly that she had to be put down. These are valid reasons for keeping cats indoors and show that, in some areas, indoors is safer. Sure, an owner can be sensible about safety, but human safety watchdogs say that most accidents happen in the home. Raising Indoor Cats, Supervised Outings Behaviourists have noted that indoor cats can become neurotic or bored if a stimulating 3-D environment is not provided by the owner. They also need more regular play since an indoor environment doesn't present the ever-changing environment found in a garden.
British behaviourists report more problems in indoor cats than in outdoor cats.
This does not apply to sexual behaviour since neutering removes the reproductive urge rather than thwarting the desire. I leave the door open and they are usually back in the house in a shot. And they rarely show any interest in going back out. Some owners take their cats on supervised outings which the cats enjoy. We take our cats out on leashes into the city fairly regularly.