What is (really) best for ferals

I have two ferals that I have adopted, Tigger, and Paxil, most of you know about them.
My big question is;Am I doing what is the humane thing by traping ferals, and having them spayed, or nutered, and releasing them to my yard, food avalible, just can't "adopt" them all with a proper home.I have two friends that are very "proper" with doing whats right, and askewing whats wrong.On the subject of cats, they tease me a little about what I have been doing, and said it would be more merciful to shoot them, than to change their nature, and let them starve by loosing their natural ability to function in the wild.
Between my daughter, and I we have nutered over thirty cats, and my heart is broken if we are doing the wrong thing.
My friends said that "bleeding hearts, usually have little forsight".I said that it looks to me like nature isn't doing too well having cats breed every time their ready doesn't produce productive offspring.LMK Mike