In a letter to council members last week, Keller said that he did not want to "put New Hope Borough, and myself, at legal risk for breach of my official duties as Mayor" by overseeing the wedding. State law defines marriage as being between one man and one woman.
In an interview Tuesday, Keller, a Republican who said he supports same-sex marriage, added that "it was a freaking heart-wrenching decision for me," but that "I've sworn an oath as I became mayor that I would uphold the laws of the Commonwealth of Pennsylvania."
The decision didn't sit well with some area residents.
"It's disappointing that our mayor did not use this as an opportunity to lead and continue the New Hope community tradition of acceptance and equality for all," council member Geri Delevich said.
Louis Licitra, a gay-rights activist who lives in neighboring Solebury Township, said that while he understands Keller's predicament, the mayor should "make the statement that although he can't do it, he disagrees with the state's discriminatory laws."
And Marcus Saitschenko, 52, the Philadelphia resident who asked Keller to officiate at his marriage specifically because of New Hope's gay-friendly reputation, put the situation in more personal terms: Keller's denial "felt like a stab in the back."
Keller's decision comes amid a flurry of activity involving the state's same-sex marriage laws.
Last month, the American Civil Liberties Union filed a suit challenging the state's ban on same-sex marriage, and Attorney General Kathleen Kane said she would not defend the ban in court. The Corbett administration is defending it instead.
Suits have also been filed by both supporters and opponents of Montgomery County's Register of Wills office, which has been defying state law and issuing same-sex marriage licenses since early August at the direction of Register of Wills D. Bruce Hanes.
Such a frenzied legal landscape played a part in Keller's decision to pass on officiating the wedding, he said, adding that the borough solicitor advised him to decline the chance to officiate.
"When you get advice from your solicitor, who represents the borough and is looking out for the borough's interest, to me it's 'shame on me' if I ignore that," he said.
Several legal experts said it was not particularly clear what legal consequences Keller or New Hope might face if he did officiate a same-sex wedding, since so much is pending in court.
"This is all new," said Pennsylvania State University law professor Jill Engle. "These are examples of cutting-edge legal issues, where the law is getting transformed before our very eyes."
What is clear is that if Keller had chosen to officiate the wedding, he would not have been the first elected official in the state to do so.
State Sen. Daylin Leach (D., Montgomery) said he's overseen about a dozen same-sex marriages this month. The wedding officiant should not be the one determining a marriage license's validity, he said.
"My role is just, if there's a license, it appears to be valid, and appears to be issued by an appropriate authority . . . then my role then is to marry that person," he said.
Keller said he appreciated that people have differing views on what stance he should take, particularly in New Hope, where the council's equality-focused actions have included a 2004 resolution urging the state to legalize same-sex marriage.
And if same-sex marriage is legalized, Keller said, he'd be "thrilled" to officiate a wedding.
But Saitschenko, who said he's been with his partner for 22 years, still didn't feel that Keller was going far enough.
"I think we need more people who stand on principles of fairness," he said, "than people who hide behind bad laws."
Contact Chris Palmer at 609-217-8305, firstname.lastname@example.org, or follow on Twitter @cs_palmer.