Why oh why do people let their cats out un-neutered? There is a house down the street that has about 5 cats and at least 2 of the toms still have their balls. 1 of them picks on all the other cats in the street (mine included) and this would, if not stop it, at least reduce his need to fight. I have a female kitten (rescued as an unwanted kitten from an accidental litter). I would never even dream of letting her out without spaying her first. If she did accidentally get out she'd probably get pregnant in about 5 minutes with all the intact males!
Just a rant really. I don't think there is an answer because there will always be idiot pet owners.