The Core Issues Trust believes that people are not born gay and that they can change.
Co-director Mike Davidson told UTV that reparative therapy should be available for anyone conflicted about their sexuality.
"I don't think gay people would want to be involved in reparative therapy and why should they? They are happy and they are contented in the life that they feel is natural and normal for them - but surely it would be inappropriate to think that one cap fits all," he said.
"Some people are just not happy with that model and really I think it would not be equitable if we did not provide support for them."
But the Rainbow Project, who organised Tuesday's protest, insists Core's message that being gay is a lifestyle choice is dangerous.
People who have been through reparative therapy in the past have experienced such things as depression, difficulty in forming relationships and internalised homophobia.
John O’Doherty, Rainbow Project
"Medical associations across the world are opposing this type of therapy - just last year, the British Medical Association took the decision to oppose this because it harms people," director John O'Doherty told UTV.
"It harms their mental health, it harms their self-esteem and it harms their ability to form relationships in the future."
According to Core, the British Medical Association and others are confusing reparative therapy with conversion therapy.
The group also says it's not suggesting gay people who are not conflicted about their sexuality need or should undertake reparative therapy.
"We don't believe that members of the gay and lesbian community should participate in such therapy," Mr Davidson said.
"Because we need to very clearly distinguish between people who identify themselves as gay and are happy and quite legitimately wish to continue in a gay relationship - we're not wanting to impose a therapeutic opportunity on them - and those who are conflicted in their sexuality."
Californian-based therapist David Pickup is the keynote speaker at the event.
Reparative therapy is based on the premise that homosexuality is not naturally inborn - that there are causes that usually always happen during those very crucial years from two until seven.
David Pickup, reparative therapist
"How can we deny any individual the right to self-determination of sexual identity and support in achieving that?" he said.
The group Changing Attitude Ireland has also voiced concern about the conference and the message it sends out.
"At a time when old prejudices against homosexuality are being questioned and abandoned, it is sad that a local parish church should be accommodating an organisation that believes that homosexuality is a life-style choice," CIA secretary Canon Charles Kenny said.
"There are genuine problems about Christians reconciling one another's views on homosexuality - as there were about slaves, the welfare state, the role of women, etcetera - and we need to work on them with dialogue which is respectful on both sides.
"But we are not helped by encouraging the destructive influence of 'ex-gay' organisations."
Belvoir Church's minister, Canon Tom Keightley, has stressed that the church itself is not involved with the running or promotion of the event or with the organisation behind it.