Jim Bennett, regional director for Lambda Legal, said supporting same-sex marriage is "both politically smart and morally right."
"I think it's a safer vote to vote with us than to vote against us at this point," Bennett said.
The issue has caused internal conflict among Republicans as the party works balance its efforts to appeal more to younger voters, minorities and women with the more socially conservative positions of some members.
After Illinois GOP Chairman Pat Brady announced his support for gay marriage during the lame-duck legislative session, saying it was a civil rights issue, some Republicans called for his ouster. Opponents of gay marriage pledged to fund primary challenges to any Republican who voted in favor of the bill.
Three Democrats joined Republicans in voting no, while two were absent and two voted present.
In addition to religious freedom concerns, opponents said Thursday they worried that the bill would change what children are taught in schools. Others said it would diminish the sanctity of marriage.
"People have a right to live as they choose," said Sen. Kyle McCarter, R-Lebanon. "They don't have a right to redefine marriage for all of us."
Supporters said it was a matter of civil rights. Several lawmakers talked about family and friends who are gay and wanting to give them the same rights and protections that same-sex couples enjoy.