Because all evil first takes place in the brain before it becomes an action. Pornography doesn't lessen desire -- know any men who say "well, I have Playboy and my 900 number, that's enough, I really don't want sex with a woman"? Constant exposure to anything desensitizes you -- look at how accustomed we have become to swearing in public, or even how tatoos on women, fifteen years ago shocking, seems normal. And, of course, you've NEVER tried a product or service because of an advertisement, right?
Images of children having sex will now be widely, legally available, rationalized because "we aren't hurting any real children." And we will become used to seeing images of children being violated and it will seem, after several years, very normal. I don't want to know where it goes from there.