Supporters of the Illinois Family Institute were advocating for what they call "natural marriage" between one man and one woman.
As they marched through the streets, they were joined by supporters of the Gay Liberation Network.
"We are not against them. We try to follow the bible and what God says about the gospel," same-sex marriage opponent Maria Elena Valdez said.
"It's kind of ironic to me that a group that says they are trying to protect marriage are actually fighting against marriage for other people," same-sex marriage supporter Jason Eiler said.
The Illinois State Senate approved gay marriage legislation on Valentine's Day.
Supporters in the house say they're still a few votes short of what's needed to pass the measure.