"Well I want sex but I don't want the baby. The baby is less important than my own personal pleasure. Some people have sex only for the pleasure and that's all that matters!"

Here's a good response (ironically found on tumblr of all places):

I have a blue button that creates kittens. You want a kitten, so you press the button. You notice many others are pressing the button and creating kittens. But then you also notice the other people who press the blue button, creating a kitten, and then they drown the kitten in a bucket of water. And they do this over and over and over. Create kitten, drown kitten. You ask why they would drown the kittens. "We don't want kittens," they say. And you wonder why on earth someone would press the kitten-creating button over and over if they didn't want a kitten.

"Maybe we just like the color blue," they respond. "Maybe we just like pressing the buttons."

"But every time you press the button, a kitten dies," you plead. "What if you didn't press the button at all? What if you pressed the button and let the kittens live?"

"We want to press the button, and we want to kill the kittens."

That's the literal definition of evil. "We want pleasure at the expense of killing people. We get pleasure from killing people."