A 22-year-old Nebraska man is charged Monday with first-degree assault for having sex with his underage 13-year-old wife.

Matthew Koso legally married the girl in Kansas after she became pregnant. Her mother gave Koso permission to take the girl to Kansas where minors can get married with parental consent. She is now seven and a-half months pregnant.

Nebraska requires people to be at least 17-year-old before they can marry.

If convicted, Koso faces up to 50 years in prison. He was released on $7,500 bail pending an Aug. 17 preliminary hearing.

Koso's lawyer, Willis Yoesel, said the couple, with their families' support, "made a responsible decision to try to cope with the problem."

He adds that the girl's mother and Koso's parents approved of the marriage.

Nebraska Attorney General Jon Bruning says the marriage is valid, thanks to the "ridiculous" Kansas law, "but it doesn't matter. I'm not going to stand by while a grown man ... has a relationship with a 13-year-old - now 14-year-old - girl."

Bruning has aggressively prosecuted sex crimes against children since he was elected in 2002