The Senate approved a bill late Tuesday that said "the parties, co-parties and accomplices" of a homosexual marriage can be punished by three years in prison and a fine of about USD 8,000.
"All senators are opposed to same-sex marriage, so this simply reflects the commitments the senators made during their campaigns," Senate President Youri Latortue told AFP.
Haiti's constitution established a secular republic but the country is marked by deep religious beliefs.
"A country has to focus on its values and traditions. Some people in other countries see it differently, but in Haiti, that's how it's seen."
More From This Section
Haitian law already defines marriage as a union between a man and a woman, making it unclear what consequences the bill, if passed, would have in practice.
However, it also called for banning "any public demonstration of support for homosexuality and proselytizing in favour of such acts."
"We see this as an attack on the LGBT community in this country," said Charlot Jeudy, president of the Kouraj group, which defends the rights of homosexuals and transgender people.
"This text divides our society, it reinforces prejudices and discrimination. It's really a shame."
The two gay rights organisations still officially recognised by the state record daily instances of insults, threats and violence.
Politicians "know very well that this will bring much more violence and prejudice against the LGBTI community," Jeudy said.
Only police and the judicial system can intervene in response to reported violations under the measure.