Ryan Miner, a 19-year-old sophomore, was found guilty of violating the school's University Code, which prohibits harassment or discrimination, Duquesne spokeswoman Bridget Fare said.
Miner must write a 10-page essay on the church's stance on homosexuality and remove the offensive post by Dec. 15, according to the sanction.
"I absolutely, 100 percent refuse to write the paper," Miner said. "I don't stand down when I believe in something."
The incident stems from ongoing opposition to a proposal submitted by gay students to form a Gay-Straight Alliance group on campus. The university formed a committee in September to examine the issue in light of its Catholic identity.
Miner started a group called "Students Against the Gay/Straight Alliance" on Facebook.com, a Web site allowing public users to post personal profiles and blogs. The Web site is not affiliated with the university.
Miner wrote that homosexuality was "subhuman" in a posting submitted on the site last month. He said he knew some people would be offended by the post, but didn't think he was violating the University Code.
"I wasn't saying the people were subhuman," he said. "I was talking about the act. If I thought I was in violation, I wouldn't have wrote it."
Students noticed the posting and reported it to school officials, who found Miner in violation of the school code.
As a private school, Duquesne has the right to limit free speech and can punish students for breaking rules even when they are committed off campus, Fare said.
"While they're students here, they're still representing Duquesne at all times," Fare said. "It's OK to disagree, as long as a viewpoint isn't degrading or demeaning to others. Calling somebody subhuman is not a respectful way to disagree...