Missouri law already bans gay marraige but some lawmakers say that's not enough.
A recent ruling from the high court in Massachusettes overturned a similar law saying it was unconstitutional.
Springfield Republican Representative Mark Wright says he fears the same thing could happen in Missouri constitution.
But opponents say the proposed constitutional amendment is discriminatory.
Democratic Representative Melba Curls spoke against the measure.
But supporters of the proposed ban say it's not anti-gay.
Republican Representative Susan Phillips.
But during debate, Republican Representative Bryan Stevenson pointed out to his colleague Kevin Wilson that there's nothing wrong with the state discriminating against certain activities.
Opponents say they don't believe the constitution should be changed in this way.
Democratic Representative Vicki Walker says it would set a precendent.
Some lawmakers assert the push for legalizing gay marriage is part of a larger gay rights agenda.
But Walker says the issue goes beyond gay rights.
The measure faces a final vote in the House before moving to the senate for consideration.