The pastor of one of Houston’s megachurches is asking Mayor Annise Parker to resign if she will not cease promoting the legalization of same-sex marriage.

“Respectfully, if you cannot uphold the Texas constitution, then you should do the honorable thing and step down,” Pastor Steve Riggle of Grace Community Church wrote in an email to Parker on Friday.

In Riggle’s view, Parker is failing to uphold the state constitution, which includes a voter-approved amendment banning same-sex marriage, by advocating for equal marriage rights for gays and lesbians.

In January, Parker joined 77 of her colleagues at the U.S. Conference of Mayors in Washington, D.C., in calling for legalization of same-sex marriage.

Her participation in the event, weeks after her swearing in for a second term in January, amounted to what Riggle described in the email as a “call for action regarding marriage that would violate the very constitution you were swearing to uphold.”

When asked about Riggle’s message, Parker said Monday, “I do my duty to uphold the state Constitution and the U.S. Constitution. I swore an oath to that. I take that oath very seriously, but I have my First Amendment rights 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,” Parker said.

Grace has 15,000 members, according to Riggle, making it one of the largest churches in Houston and in the nation. It is also where the Harris County Republican Party plans to have its convention in April. Parker is a Democrat.

Riggle also wrote that he was “deeply disturbed” by a city memo that identified the mayor’s longtime domestic partner as the first lady of Houston “since you know that title has been given only to the wife or husband of the mayor in the appropriate gender language.”

He additionally asked the mayor not to use her office to promote her personal views and to support traditional marriage as expressed by the voters.

“I’m going to continue to follow my oath of office, lead the city well but speak out on issues that I care about,” Parker said Monday.