A judge today denied a request for a lighter sentence and ordered a Lawrence man to 21 years in prison Thursday for having sex with his children’s 13-year-old babysitter.

“She was vulnerable. She was seemingly desperate for affection and acceptance,” District Court Judge Paula Martin said as she imposed sentence on William R. Rivera, 36. “Mr. Rivera preyed on those vulnerabilities. He used her and exploited her.”

A jury in October found Rivera guilty of having sex with the girl three times in a relationship that lasted about a month beginning in May 2004. Sex with a child under 14 is classified as rape under Kansas law.

Rivera’s attorney, Greg Robinson, had asked Martin to deviate from sentencing guidelines by disregarding Rivera’s criminal history. That would have given Rivera a sentence of 155 months, or nearly 13 years, according to the state’s sentencing grid.

But Martin found there weren’t sufficient reasons for a lighter penalty and gave him the presumed sentence of 253 months, or roughly 21 years.