Christian Concern is encouraging supporters to write to Channel 5 to express their disappointment over its treatment of Evander Holyfield in Celebrity Big Brother 2014.
The former heavyweight boxing champion was the latest contestant to be evicted from the Big Brother house.
During his time in the house, he disciplined by Big Brother bosses for making what they deemed to be 'homophobic' comments after he suggested homosexuality was a disability that could be "fixed".
His comments came about when fellow housemate Luisa Zissman asked whether any boxers had come out as gay.
During their conversation, Zissman said: "I think it's really bad that no-one stands up and says it in the sporting industries. I think it's good to be open about that because it's normal."
In response, Holyfield said: "What would be good about it? That ain't normal … The Bible lets you know, that's wrong, that's right."
The 51-year-old went on to say: "It is a choice, come on, that ain't the way nobody is made.
"If you were born and your leg were turned this way, what do you do, you go to the doctor and get it fixed back right."
Holyfield was called to the diary room where Big Brother reminded him about house rules regarding "unacceptable language and behaviour".
"While Big Brother understands these are the views you hold, they aren't the views that are held by a large section of society and expressing these views will be extremely offensive to many people. Do you understand why?"
Holyfield replied: "Yeah I understand why, I forgot about the thing. I was just telling her my opinion, but it's not like I was gonna mention it to anybody else, that was just our conversation."
Big Brother then went on to warn the boxer: "Big Brother does not tolerate the use of offensive language and must therefore warn you to consider very carefully the effect expressing such views may have and the harm and offence you may cause by repeating these views inside the house."
Advocacy group Christian Concern said Big Brother's reaction was "stifling freedom of speech" and "censorship of Christian truth.
"Channel 5 censured Evander Holyfield for gently expressing his viewpoint in the context of a measured conversation, suggesting that reasonable discussion of the issue is not permitted," it said.
Christian Concern also accused Channel 5 of "double-standards".
"Channel 5 rebuked Evander Holyfield for his views on sexual ethics but has previously broadcast highly provocative material (e.g. two women kissing in a hot-tub)," it said.
Letters can be sent via email to email@example.com or posted to: Channel 5 Television, 10 Lower Thames Street, London EC3R 6EN