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

“She was vulnerable. She was seemingly desperate for affection and acceptance,” District Court Judge Paula Martin said as she imposed the 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.

Assistant Dist. Atty. Dave Zabel credited two children who knew Rivera and the victim for being mature enough to realize something was wrong and come forward to authorities.

“But for their courage, the truth may not have come out,” Zabel said.

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.

“We’re not asking the court not to punish Mr. Rivera,” Robinson said. “In fact, 155 months in the Department of Corrections is a long time.”

A Lawrence woman who considers Rivera a father figure testified he was a good father and grandfather who cared for and supported his family.

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.

Martin also denied Rivera’s request for a new trial, finding there wasn’t evidence to support his allegation that one of the jurors told another person Rivera was guilty before all the evidence in the case had been presented.

Martin was under fire last year for granting lightened sentences in a separate statutory rape case involving a 13-year-old.