Cats left outside for the winter

Sassy (the one on the left who is all black) is my only outdoor cat. On his way outside last night, he was greeted by 3 other black cats. That surprised him a bit, but, undaunted, he went out.

I looked for him later, and saw a 4th cat, a beautiful striped Maine Coon. He doesn't come much.

I look for the outside cats regularly, because I'm concerned about their ability to stay alive in winter weather. While the weather is mild this year and the cats don't seem to be suffering, what might happen to them if it were cold?

My point is that if someone owns a cat, they have the responsibility of taking care of that cat. That includes spaying or neutering. I feel strongly that not doing this is a form of animal cruelty.