Rauner already stirred controversy with his conservative base when he signed a bill to provide taxpayer-funded abortions in Illinois.
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Rauner officiated the wedding of his friend, Mark Cozzi, on June 30. Cozzi said their paths crossed in the finance world in the 1990s.
"Diana and Bruce are wonderful people and they're great friends and we thought it would make our special day even more special if the governor was our officiant," Cozzi said of the wedding.
The wedding picture, posted on Instagram, was considered a slap in the face by many social conservatives and Evangelical Christians.
Many Republicans had already felt betrayed when Rauner signed HB40 to provide taxpayer funded abortions after promising he would not.
"What I'm hearing through our connections and our networks throughout, people are once again saying there he goes again poking social conservatives in the eye," said Executive Director of the Illinois Family Institute David Smith.
Rauner has been very supportive of LGBTQ rights as governor, most recently by participating in Pride parades in Aurora and Chicago.
Many Republicans said officiating a same-sex marriage should be a non-issue for voters.
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"We should put aside some of our differences, because there's so much more that we agree on, the governor, myself and also conservatives and independents throughout the state of Illinois," said Illinois House Republican Leader Jim Durkin.
"Well, I would hope that voters would appreciate that Bruce is very much in favor of equality of all people," Cozzi said. Cozzi added if the picture helps voters appreciate Rauner for his stance on equality, that's a good thing.
But for Rauner, rallying conservatives just may not be in the cards.
"I don't think there's much he can do to win us back, to win many social conservatives back," Smith said.
The governor's office said the wedding was a personal event, and Rauner's campaign spokesman wondered why anyone would make a big deal of it.
Rauner also officiated a same-sex wedding in December 2017.