No, it's not required...but it's truly the right thing to do. In fact, I would venture to say that it is the single most important responsibility you have as the owner of a cat.
I don't want you to think that I'm jumping down your throat, because certainly I'm sure you love your cats and kittens, and you probably believe what you're doing is the right thing. I'm going to go ahead and post some things I think you should read...because believe me, the shelters are populated enough with unwanted cats and kittens, and something needs to be done. Fortunately, the solution starts when each and every owner makes the decision to alter their pets.
I'll go find some things for you, and I encourage you to read them. I know you didn't intend for your females to become pregnant...it happens quite naturally when they're outdoors and not spayed. But the operation itself is not that costly, and it actually has a lot of health benefits for females...such as eliminating ovarian cancer down the road, for example.