Gambling sites really do prey on stupid people, but why shouldn't the government protect stupid people too? Perhaps blocking sites is too much, but I would support, for example, a mandatory government warning, and a hefty tax. Like cigarette taxes and the warnings that appear on cigarette packages about lung cancer, for example. You are free to do this at your own risk, but you should know what that risk is. This is your chance of winning, compared with your chance of being struck by lightning. This is the money you could save in one year and the return on it after 20 years in the bank.
Pornography and prostitution (the two are hard to separate) are worse for several reasons. One is that consumers of sex for money are socialized to see people as objects, things to be used. That's unethical in any religion or moral system. Men who buy sex (with a very few exceptions such as the disabled) are more likely to be abusive in other ways.
Sex trafficking, selling people as sex objects (whether in porn or prostitution), is also wrong because real people are damaged -- it's not by any means a victimless crime. The majority of those who work in the sex industry are not there by free choice. Studies are just beginning to show how many of them were abused or neglected as children, leading to a cycle of dropping out, running away, becoming addicts, and depending on pimps for food, shelter, drugs, and 'love.'
Coincidentally, there was just a film on the PBS series 'Independent Lens' with a very sobering look at the lives of women caught up in sex trafficking. In the US, mark you, not in India or Thailand.http://www.pbs.org/independentlens/path-appea...
Part of the solution is to provide shelters and support for victims who want to get out of that cycle. Another part is to crack down on johns, the men who buy sex and visit online porn sites, especially ones featuring underage victims. And if another part is to block certain sites (as opposed to words) in search engines, why not?
I don't actually believe that modern internet technology is still so primitive that it can't distinguish between words like Scunthorpe and Essex and porn sites.
I don't think every individual parent ought to have to spend hours trying to set up internet filters and figuring out how to block every new piece of malware or trick of deceptive advertising.
And I don't think it ought to be so easy for people to victimize other people for sex, even if it's 'only' online. Maybe one day people will be able to see their fantasies enacted by virtual characters; I don't know if that would be a good thing or not. But for now, most of the victims in the pictures are still real.