Falls City, Neb. ? A 22-year-old Falls City man faces up to 50 years in prison after pleading guilty Tuesday to a sexual assault charge filed after he impregnated and then married a 14-year-old girl.

Matthew Koso legally married the girl, who turned 15 last week, in Kansas seven months ago after she became pregnant. The girl, Crystal Koso, gave birth Aug. 24 to a 7-pound, 1-ounce girl, Samara.

Sentencing was set for Feb. 7. Koso originally had pleaded not guilty in August.

Koso left the courtroom with his wife and declined to say why he changed his plea.

His attorney, Willis Yoesil, said he had advised Koso to change his plea but would not say why. But Yoesil did say that he believed Koso had a chance at getting sentenced to probation.

Later Tuesday, Atty. Gen. Jon Bruning said his office would not be making a recommendation to the judge about Koso’s sentence.

“Mr. Koso committed a serious crime, and he must take responsibility,” Bruning said.

Bruning, who was criticized by some for filing the first-degree sexual assault charge given that the sex was consensual and the couple eventually married, defended his decision to pursue the case.

“This office, and I personally, have a responsibility to protect Nebraska’s children,” Bruning said. “There was a clear violation of the law in this case, and I could not ignore that.

“The law clearly states that it is illegal for a grown man to have sex with a child,” he said.

Richardson County District Judge Daniel Bryan ordered a pre-sentence investigation, which would include an evaluation of Koso’s mental capacity. The judge repeatedly quizzed Koso about his decision and whether he understood he could spend time in prison for the crime.

“Are you telling the court that you fully comprehend what you’re doing today?” the judge asked.

“Yes, sir,” Koso said.

The case generated national media attention when Bruning filed the charge against Koso in July. Koso’s trial was scheduled to begin on Thursday.

Assistant Atty. Gen. Matt Enenbach told the judge Tuesday that the evidence would show that Koso was the father of the child and that Koso had had sex with Crystal when she was 13 years old.

Nebraska requires people to be at least 17 before they can marry. State law also prohibits people 19 or older from having sex with those under the age of 16.

After the girl became pregnant, her mother gave permission for Koso to take her daughter across the border to Hiawatha, Kan. There is no minimum age for marriage in Kansas, but state law requires anyone under 18 wanting to marry to have permission of both parents or a guardian. If there’s no parent or guardian, permission from a judge is required.

Koso’s lawyer, Yoesil, has said that the girl’s mother and Koso’s parents approved of the marriage. The girl’s father has not lived with the family for some time.

Koso’s wife, who was in the courtroom along with other relatives, openly wept during the 25-minute proceeding.

Kansas Gov. Kathleen Sebelius recently said she was asking legislators to establish a strict ban on marriage involving anyone younger than 16.

The governor said the legislation also should retain the requirement for parental or judicial consent for those age 16 or 17.