I have to say that if my cats had been happy to stay in doors that I would not have allowed them out. It is actually a lot safer for them and if they are happy with an indoor existence and you can provide them with lots of 'enrichment' activities, then it is more sensible to keep them that way. Apart from anything else it will save you a fortune in vet bills as they will not be exposed to fights, disease, risk of car accident etc. I say this having two indoor-outdoor cats at present, but if you look at the thread that JS has started (somehing along the lines of 'reasons for indoor/outdoor cats') you will see that a lot of cats live rich, fulfilling and long healthy lives when kept indoors. It is a nonsense to believe those who tell you you are thwarting their 'natural instincts' as these can be easily met with the right toys, and interactions with their humans. A similar argument is used to justify keeping cats intact, and you know what a nonsense that idea is!