In any case, I don't think it would be effective to opt out. It's not like your child will opt out of the school yard talk that's going to happen after the class discussion. They may as well hear it first-hand without miscommunications, poor translations and rumours.
Some people have no choice to put their children in public school. What if I don't agree that certain things involving sex should be considered "normal"? If my son hears something on the schoolyard, it's different than having a teacher, an adult, telling them "not only is it normal and okay, but why don't you go try it as well"? My son heard about fisting from his teacher, but he heard about gangbangs and beastiality from his friends- should that be taught in class, too? SOME people consider that normal, so why don't we teach them how to do THAT "safely" and "properly"? Where do we draw the line, where does the parent get to parent his or her child in what her family values are? The whole point of parenting is to first teach your children your views, your values, and to have someone else force you to teach them otherwise is against everything you believe in. Everybody wants to say they're so openminded about sex that they're pretty much "everything goes", and forgetting that sex education, like all other things, should be the parent's choice how it's taught. If someone was a strict vegetarian, would they be okay with the schools insisting that their child needs to hunt, kill and eat meat? If someone never laid a hand on their children, would they be okay with the schools using corporal punishment on their child? If someone was an Atheist, would they be okay with their child having "religion class", because they might hear about it on the schoolyard?