The website is part of MoveOn.org Civic Action, a progressive nonprofit.
"I've never done anything like this, and it's kind of amazing how fast this took off," said Paul Krueger of Murray, who started the petition.
Krueger, who said he's a retired firefighter who now drives school buses, said he didn't think the bill, HB363, would actually pass because it was "ridiculous." When it did, he thought maybe he could "make a difference."
"I just believe that it's the wrong thing to do," Krueger said. "I just think that taking away a chance to learn about sex education, sexually transmitted diseases, any of those things, contraception ... I don't know where they're going to get the information."
Parents may already choose to have their kids skip sex education classes under current Utah law, but few do, according to a Salt Lake Tribune survey of Utah's larger districts.
Krueger said he first posted the petition midday Wednesday, letting people know about it through his Facebook page. Word spread via e-mail, Facebook and Twitter.
Liz Zentner, Utah PTA president-elect praised the petitions on Thursday. The Utah PTA had opposed the bill.
"It's going to be very, very tough for him to make this decision, and I think the more he hears from the public, the better it will be," Zentner said of the governor. "I hope he does pay attention to what people are saying because there's been an overwhelming wave of public outcry on this bill."
Gov. Gary Herbert said Thursday he had not yet made a decision on the bill. He said "the intent here really is that parents and guardians should have the foremost authority of giving sex education to their children. And I realize families are different but the primary focus should be on parents and them taking on that responsibility."
Ally Isom, spokeswoman for Herbert, said the Governor's Office has received more than 900 letters, emails and phone calls from people opposing HB363.
The bill's supporters, however, said Thursday they were not fazed by the online signatures. Proponents of the bill have argued that sex education doesn't belong in schools and that, if they do teach it, they should teach abstinence-only because it's the only sure way to prevent teen pregnancy and disease.
Bill sponsor Rep. Bill Wright, R-Holden, said the fact that some Utahns oppose the bill is not surprising, given that some lawmakers opposed it, and those lawmakers have constituents. The Senate passed the bill 19-10 and the House passed it by a margin of 45-28.
He said he doesn't believe the governor should veto it. He said he looked at "what's in the best interest of the state, the future of the state," in running the bill.
Gayle Ruzicka, president of the Utah Eagle Forum, which also supported the bill, said "you don't veto by poll." Ruzicka also questioned the validity of an online petition.
"Online petitions, you can get that many signatures from people all over the country," Ruzicka said. "Do we know that everybody lives in Utah? Do they have their addresses on there or is it just their names? If it's just names that they're going for, I could get more than that by tomorrow."
The SignOn.org petition asks for addresses but seems to require only names, email addresses, countries and ZIP codes to sign.
According to the National Conference of State Legislatures, as of February 2011, 21 states and the District of Columbia required public schools to teach sex education, and 35 states and the District of Columbia required students to receive information about sexually transmitted infections.
Robert Gehrke contributed to this story.