The preventative (aka deterrent) nature of a prophylactic is moot unless the adversary it is designed to thwart occasionally gets to see it do what it was intended to do.
When I was in Kosovo in 2002, the locals (Orthodox Christians and Muslims) were not killing each other, though they certainly had been before the arrival of armed force. They sometimes expressed their desire to see US and NATO troops depart, saying it would now be ok for them to do so since the fighting had stopped. The trouble is, as we have seen so many times, once the troops leave, the killing resumes. Why? The very presence of troops with guns keeps things in check. Remove that prophylactic, and the trouble that's been held in check is free to flourish again.
In the United States, the citizenry is armed, and that's not by accident. The Founders knew that arms are the ultimate preventative, and they knew that if one removes the lid from a box of cute, cuddly kittens, the cute, cuddly kittens leave the box. OK, so such kittens are a manageable problem. But what if the kittens were not so cute and cuddly? What if they quickly grew into lions and tigers hell-bent on taking over your household?
Armed citizens are only lid on the box of Government kittens. They claim to be cute and cuddly, only trying to help us lead easy and care-free lives. But they're really lions and tigers, determined to stand on our chests with teeth and claws bared.
Do we want them out of the box?