The diocese says Shaele Evenson was fired because she didn’t "follow a lifestyle in line with Catholic teachings."
Evenson had been with the school nine years, teaching PE and literature to kids from 6th though the 8th grade.
Butte Central Catholic Elementary School Superintendent Patrick Haggarty told KXLH Evenson, "made a willful decision to violate the terms of her contract."
"The Catholic moral teaching is that the sacrament of marriage is a holy union between a man and a woman. And we certainly believe, and we teach our children who attend our schools about the sacrament of marriage."
Haggarty says contracts demand all employees follow Catholic teachings in their personal and professional lives.
“It’s a sensitive issue, and it’s unfortunate all around,” Haggarty added.
The Montana Standard reports the woman's attorney is preparing a civil rights charge the school.
Some Catholics in Butte, Montana are against the firing.
They point out Pope Francis baptized the child of a set of unmarried parents last month.