Apple Senior VP of Marketing, Phil Schiller, was quoted by the New York Times in an article on the removal of 5000 sex-based app from the iTunes App Store:
“It came to the point where we were getting customer complaints from women who found the content getting too degrading and objectionable, as well as parents who were upset with what their kids were able to see.”
To developers who weren't afforded any warning or options to pre-emptively make changes where such changes would have been possible:
“We obviously care about developers, but in the end have to put the needs of the kids and parents first.”
As to why Sports Illustrated's Swimsuit app, the Playboy app, and a few other publication-associated apps were allowed to remain:
“The difference is this is a well-known company with previously published material available broadly in a well-accepted format.”