TORONTO - Ontario's Catholic schools can't opt out of a revamped sex ed curriculum even if it goes against their religious beliefs to teach kids about masturbation and homosexuality, Premier Dalton McGuinty said Wednesday.

Several conservative and religious groups claim the changes coming this fall will corrupt young minds with "explicit" topics like anal sex. But Ontario's Catholic premier made it clear that all public schools must teach the lessons that will start as early as age six.

"They're part of the publicly funded school system here in Ontario and this is part of our curriculum," McGuinty said.

"If parents are uncomfortable with certain aspects of this new curriculum, they can and they are free to withdraw their children from the classroom."

The first changes to Ontario's curriculum in a dozen years will see students learning about masturbation in Grade 6 and oral and anal sex at age 12.

Opponents who are mounting a campaign to get rid of the program have described the curriculum as "evil" and "bordering on criminal." They say teaching eight year olds about gender identity and same-sex marriage is inappropriate and should be left up to parents.

But McGuinty -- whose wife is a Catholic school teacher -- said it's a responsible way to teach kids about sex in an age where it's easy to access information, whether it's from their friends or the Internet. He argues that by including it in the curriculum, the government has some control over the information and can present it in an appropriate way.

Education Minister Leona Dombrowsky, a former Catholic school board trustee, noted that Ontario's Catholic bishops have endorsed the changes. But she wouldn't say whether schools will lose funding if they refuse to teach the new lessons.

"We do expect that schools will work with parents, and if parents would say that there is part of the curriculum that they do not want their child to receive, then they have the right to make that choice and work with the classroom teacher to accommodate that," she said.

As the mother of four grown children, it was important to her that they had the "best and correct information on any and every issue," Dombrowsky said.

"I think the reality is today that children are exposed to a wide range of mediums, more so than when our children were small," she added. "We did not have the Internet when our children were small."

Under the changes that were quietly released in January, Grade 1 kids will be taught to identify genitalia -- among other body parts -- using the correct word, such as penis, vagina and testicle. The 1998 curriculum made no mention of genitalia.

Grade 3 lessons about the differences that make each person unique will now also include discussion about same-sex families and students with special needs to "reflect the government commitment to equity and inclusive education," according to provincial officials.

In Grade 5, kids will be taught to identify parts of the reproductive system and describe how the body changes during puberty. In Grade 7, they're taught how to prevent unintended pregnancy and sexually transmitted infections, including HIV.

The Opposition Conservatives say McGuinty deliberately kept the new curriculum under wraps, knowing it would be an explosive issue for many parents.

"I mean, Dalton McGuinty does a press release when he has a good hair day," said Tory Leader Tim Hudak, who has a two-year-old daughter.

"They do press releases about everything under the sun, but somehow these fundamental changes to the sex education curriculum -- that will see kids as young as six years old getting sex ed -- they simply tried to slip by everybody."

The changes "don't sit right" with the vast majority of Ontario parents and McGuinty should hold off on the changes until he gets more feedback from them, Hudak said.

But McGuinty and Dombrowsky insist parents and experts had their say during the two-year consultation process.

Not everyone opposes the changes, said New Democrat Cheri DiNovo, a former United Church minister.

"The gift of sexuality and the gift of body parts from God is a gift," she said. "It's a gift all children need to learn about; we all need to learn about."