Northrup issued a statement through his school expressing his "tremendous" respect for what Herkelman and Ottumwa sophomore Megan Black achieved this season, but he said didn't feel he had a choice.
"Wrestling is a combat sport and it can get violent at times," Northrup said in a statement released by his high school. "As a matter of conscience and my faith I do not believe that it is appropriate for a boy to engage a girl in this manner. It is unfortunate that I have been placed in a situation not seen in most other high school sports in Iowa."
His father, Jamie Northrup, told The Associated Press later Thursday that his son struggled with the decision.
"He's poured his heart and soul into wrestling and into being the best in the state," Jamie Northrup said. "He's never won a state championship, so he's certainly looking forward to that day. So it's agonizing, from all the work and the effort and the hope.
"But it's easy in that, he, a long time ago, drew a line and said 'I don't believe it's right for a boy to wrestle a girl."'
There were several thousand fans at Wells Fargo Arena on Thursday, but many were watching other matches when the referee raised Herkelman's hand to signal her win. There was a smattering of cheers and boos from the crowd before Herkelman was whisked into the bowels of the arena.
Northrup's decision to default put Herkelman in the quarterfinals in the 112-pound weight class, and it put her name in the record book as the first girl to win an Iowa state tournament match. But it deprived her of the chance to show the skills that earned her a 20-13 pre-tournament record.
Tournament organizers declined to make Herkelman available for questions. But her father, Bill Herkelman, told the AP via text message that he understands and respects Northrup's decision.
"It's nice to get the first win and have her be on the way to the medal round," Bill Herkelman wrote. "I sincerely respect the decision of the Northrup family especially since it was made on the biggest stage in wrestling. I have heard nothing but good things about the Northrup family and hope Joel does very well the remainder of the tourney."
Because he defaulted and didn't forfeit, Northrup was allowed to compete in the consolation rounds, and he won his first match later Thursday by major decision.
He was spared any chance of meeting up with Black -- who also wrestles at 112 pounds and was 25-13 entering the tournament -- when she was eliminated after being pinned in both of her matches.
Northrup and Herkelman would be matched up again if both were to make the finals in the consolation bracket. If that happens, Northrup would likely make the same decision, his father said.
Jamie Northrup is a minister in the Believers in Grace Fellowship, an independent Pentecostal church in Marion that believes young men and women shouldn't touch in a "familiar way," said Bill Randles, the church's pastor.
"We believe in the elevation and respect of woman and we don't think that wrestling a woman is the right thing to do. Body slamming and takedowns, that full contact sport is not how to do that."
Randles said Joel has been involved in wrestling for many years, and he and his family have discussed before the possibility of girls getting involved in the sport.
"It's totally his choice. He's a young man now and he's worked hard to get where he's gotten. It's up to him, and it was his conviction" not to wrestle Herkelman.
Black said Northrup refused to wrestle her three years ago, and that she respects him for adhering to his beliefs.
"If it's his religion and he's strong in his religion, then I just respect that," Black said. "Obviously, everyone can be pointing fingers at him. He, at least, is true to his beliefs and you have to respect that. It takes a lot for a 15- or 16-year-old boy to do."
Marth Stetzel, a mother from Perry who had two sons in the tournament, said she had no problem with Northrup's decision.
"We're really raising kids that are going to be bigger than wrestling, and if it's something that he believes strongly in -- which is not necessarily what I would do -- you've got to respect a kid like that," Stetzel said.
Wrestling is extremely popular in Iowa, and Black and Herkelman are the first girls to qualify for the state tournament since it was first sanctioned by the Iowa High School Athletic Association in 1926.
According to the National Federation of State High School Associations, more than 6,000 girls competed in wrestling in 2009-10 -- compared with nearly 275,000 boys. Though most states require girls to wrestle boys, California, Hawaii, Texas, Washington and Tennessee sponsor girls-only high school wrestling tournaments.