JMM president Azwanddin Hamzah, who described the spread of such activities as a "planned movement by certain individuals", said the assembly was aimed at voicing out against those willing to compromise religion for political interests.
"We do not discriminate against individuals who may have been born with such tendencies. However, we are against groups that are out to make this an acceptable lifestyle.
"There is an agenda among certain individuals who clearly wish to destroy the nation's moral and religious values.
"We must put a stop to this," he said at a press conference attended by leaders of various NGOs.
Azwanddin said the peaceful assembly would be held at Dataran Merdeka on April 21, a week before the proposed Bersih 3.0 rally.
"Unlike the organisers of Bersih, who are protesting for political reasons, we are protesting for the future of our nation.
"We are expecting a large turnout. We are confident of mobilising people from all over the country to take part."
Federation of Malay Students Union president Jais Abdul Karim said although every individual had freedom of expression, Malaysia could never accept LGBT and free sex culture.
"Through this peaceful assembly, we hope our voices will be heard that such practices should not be condoned, let alone by prominent figures in the country."
On whether the non-governmental organisations would be granted a permit to assemble at Dataran Merdeka, Waris Malaya president Sheikh Weshal Sheikh Ahmad hoped the police would be fair.
"If they are going to grant Bersih permission to protest, then there is no reason why we should not be given the same treatment," he said.