Oct. 23–As a strike by 2,500 Marriott hotel workers in San Francisco approaches its fourth week and the giant Oracle OpenWorld conference begins, the bills are adding up for businesses.

The day before the 1,000-person ComNet 2018 conference earlier this month in San Francisco, the organizers scrapped months of planning and changed venues.

The strike posed a moral challenge for the event's organizer. The Communications Network, which works with foundations and nonprofits, didn't want to hold events at the Westin St. Francis while workers were marching on a picket line.

"We wanted to respect the dignity of the workers," said Sean Gibbons, the organization's CEO. "From a values perspective, it was a really easy call."

Events scheduled for Oct. 10-12 were moved to the nearby Curran Theatre and Hotel Nikko.

It came at a high cost: The Communications Network paid $300,000 to change locations. It may have to pay hundreds of thousands of dollars in additional charges related to moving out of the Westin St. Francis, Gibbons said.

The total cost of the event has ballooned from around $1 million to $1.3 million to date and will rise ever further, he said. Last year, the conference made around $200,000, but now it stands to lose hundreds of thousands of dollars.

"We've never taken a loss from the conference. We will this year," Gibbons said. "We have lost a substantial amount of money."

Still, he doesn't regret it. Gibbons credits the organization's diverse board, which is half women and includes people of color, for committing to doing what it thought was the right thing.

Since September, workers have been marching and chanting from 5 a.m. to midnight outside seven Marriott hotels in one of the largest hotel strikes in San Francisco in decades.

"It's time Marriott learned that it can't run its hotels without us," Anand Singh, president of hotel union Unite Here Local 2, said in a statement.

Hotel workers are demanding higher pay, lighter workloads and better protections for harassment from guests, pitting themselves against the world's largest hotel operator. The strikes now affect 7,700 Marriott workers in San Francisco, Oakland, San Jose, Boston, Detroit, Honolulu and Maui.

Conferences are moving out of San Francisco Marriott hotels and guests are complaining about reduced services amid the continued strike.

Workers held up signs Monday about the strike at the entrance to Moscone West, which hosts Oracle OpenWorld events this week. Oracle did not respond to a request for comment.

The Human Rights Campaign moved its Oct. 20 auction and gala from the Westin St. Francis, according to its website. Shanti Project, Chicana Latina Foundation and Bay Area Wilderness Training have also canceled business with Marriott.

"Whenever you have a labor interruption, it's a real problem," said Rick Swig, a hotel consultant whose family built the Fairmont Hotel.

He said he believes many of the conference organizers are leaving Marriotts not because they support the workers, but because they don't want guests to have a bad experience.

"I don't think it's a pro-labor gesture. I think it's a convenience issue," he said. "It's much easier if possible to move it."

The Communications Network kept its hotel room agreement with the Westin St. Francis, because it wanted to let guests with reservations keep the discounted rate rather than scramble to find alternatives. Rooms were $325 per night at the Westin and as much as $700 per night at other hotels that week, said Gibbons.

The conference booked 1,700 room nights, or roughly 560 rooms for each of the three nights. Only around a third of those rooms ended up being occupied, with most attendees choosing to change hotels. Around 90 registered attendees didn't come to the conference at all.

Gibbons never entered the Westin St. Francis, but attendees told him that there was reduced cleaning and food service. They also heard noise from the striking workers outside.

"We immediately activated our contingency plans when those strikes commenced earlier this month and have continued to operate these hotels since then," a Marriott spokesman said in a statement. "We continue to believe that the best place to resolve these issues is at the bargaining table. We are disappointed with some of the tactics the union has deployed including the noise levels. We continue to work with local police to address these issues and we are grateful to our guests for their patience during this time."

ComNet conference events went smoothly at the new venues, including talks by Lena Waithe, the actress and producer, and conservative author David Frum.

One of the most prominent speakers, Cecile Richards, former president of Planned Parenthood, is a former union organizer herself.

"She wasn't going to cross the picket line," said Gibbons.

A few days before the conference started, some of the Communications Network staff went to San Francisco's Chinatown to do some sightseeing. They bought some fortune cookies. One of them read: "Your principles mean more to you than any money or success."

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf