The "Marriage Alliance" claims Channels 7 and 10 have refused to run TV advertisements created by the group against gay marriage.
The Alliance unveiled its campaign last week with the slogan "there's more to it than you think".
Campaign spokeswoman Sophie York said today she was outraged that Channels 7 and 10 had allegedly refused to run the ads.
"It is quite shocking that two major TV networks are denying the basic right to freedom of speech and expression on an issue that supports the current law of the nation,” she said.
Channel 10 has not confirmed the claims, telling SBS: "As company policy we do not talk about our commercial arrangements, out of respect for our advertisers."
A spokesman for Channel Seven said the network had not refused to run the ads and said the the requested run times were "unworkable".
Ms York said the Marriage Alliance wanted to find out what impacts legalising same sex marriage would have.
“We so have to find out what ramifications there will be. What will be the effects, there'll be so little discussion about this,” she said at the launch last Sunday.
The group said it had contacted Prime Minister Tony Abbott and Communications Minister Malcolm Turnbull to make them aware of the decision.
"Has political correctness or the power of a certain lobby group reached so far down that it now erodes the once proud Australian 'fair-go' character that it is preventing ordinary Australians from having a voice?” Ms York said.