Houston Mayor Annise Parker is being called on to resign for promoting the legalization of gay marriage, the Houston Chronicle reported.

Parker is among the 90 mayors to join Mayors for the Freedom to Marry, an initiative of Freedom to Marry.

During a recent appearance on SiriusXM Radio's Michelangelo Signorile Show, Parker told host Michelangelo Signorile that he believes President Obama is not where he should to be on the issue.

Obama, who supports civil unions for gay and lesbian couples, has said he is “evolving” on marriage.

The president “needs to evolve a little bit faster and is not clearly where I think he ought to be [on the issue of gay marriage,]” she said.

Her backing of the issue has prompted Pastor Steve Riggle of the 15,000-member Grace Community Church to call for her resignation.

“Respectfully, if you cannot uphold the Texas constitution, then you should do the honorable thing and step down,” Riggle wrote in an email to Parker.

In response, Parker told the Houston Chronicle that she takes her oath very seriously, “but I have my First Amendment right to free speech.”

“We all have the right to do that and I'm sorry that they [Riggle and his supporters] don't understand the Constitution.”

She expanded on her detractors during her radio appearance.

“Okay, I'm an out lesbian. They know that. I'm in a long-term relationship. And yes, I want the ability to marry my spouse. … It [joining Mayors for the Freedom to Marry] provoked this wave of hate mail. … [B]ecause I'm part of that, it's like, somehow in people's eyes down here, I've changed from my role as mayor of Houston into [a] lesbian activist. Well, I'm the mayor of Houston first, but I'm still a lesbian. And I care.”

(Related: Annise Parker overcomes anti-gay attacks to win second term as Houston mayor.)

Houston is the nation's fourth largest city behind New York, Los Angeles and Chicago.