Garry Trudeau talks about masturbation and editing in a Salon.com article.(Registration required, unless you watch an ad for a free day pass.)

Technically, the exclusion of my strip from a newspaper is not censorship. It's called editing. Newspaper editors have a right and responsibility to control the content of their papers.

And why "Doonesbury" is being "edited" from papers less:

The bar got lowered with raunch radio, "South Park," and "Tonight Show" jokes about fellatio. Against that backdrop, "Doonesbury" no longer seemed quite so shocking.