WATCH: John Oliver & Celebs Tackle Sex Ed

Editor's note: the video in this story may be NSFW.

Sunday night'sJohn Oliver Tonight looked at the topic of sex education in American schools and found it lacking. The late-night host pointed out how the lack of comprehensive sex ed is not only leaving kids ignorant, it's actually harming them.

"Parental anger over sex ed is as much a staple of the school year as square pizza and one kid coming back from summer vacation with a mustache he's way too proud of," the comedian says. But "teaching sex ed in school is really important, for obvious reasons. No parent wants to talk to their kids about sex, and no kid wants to talk about sex with their parents."

Only 22 states require sex-ed classes in schools, and only 13 require that the information taught be medically accurate. In some states, decisions are left up to localities and state authorities have no idea what is actually being taught.

Most schools teach abstinence as a preferred birth control method and don't teach anything about gay or lesbian sex. Until recently, the "no sex before marriage" mantra left gay and lesbian teens without any options for having sexual relations, thanks to bans on same-sex marriage. The confusing messages combined with the lack of education left LGBT teens more vulnerable to sexually transmitted diseases, including HIV.

Several celebrities join Oliver at the end of the segment for a sex ed video for undereducated kids that's frank and funny. And again, thanks to some explicit language and anatomical drawings, it may not be safe for work, depending on the rules where you're employed.

John Oliver Tonight airs Sundays at 11 p.m. Eastern on HBO. Watch the video below.