When you’re a sex researcher with a sense of humor – which I am – and you have a history of writing about your kid’s rather lame public school sex education – which I do – and he asks you to come to his high school class to see the “abstinence-based” sex education he’s getting now – which he did – you should probably say you have a conflicting doctor’s appointment.

Instead, I went to class.

This was last Wednesday. I trust my kid to tell me the truth about what’s going on at school, but I’m really glad I saw it for myself. I also don’t regret having live-tweeted it and accidentally making my son’s East Lansing (Michigan) high school class the planet’s most talked about class.

But we’ll get to that in a minute.

Two visitors had been brought in to teach that day. I’m calling them “Ms Thomas” and “Jerry”. They didn’t reveal that they’re from a local Christian pro-life group that counsels against abortion. Whether the school knew still hasn’t been established. They just said they were from the “Smart Program”. That made them sound ... smart.

But when my 14-year-old son tried at the outset to show Ms Thomas the information he decided to bring, about how studies show that abstinence education doesn’t reduce unwanted pregnancy or disease transmission, she brushed him off and pointed him to her favorite website. That turned out to be a political group that lobbies for the funding of abstinence education.

She then introduced Jerry. Jerry told the kids about how terrible his life was when he was having sex before marriage. He had drug problems, he knocked up two different girls, and he became a less-than-ideal father to both children. He told the coed class – ages 14 through 18 – about how he knows that if a teenage girl gets pregnant, all her friends will leave her. You won’t finish school, your life will be awful.

Jerry told the kids about how he finally met a woman who lived “the abstinence lifestyle”. He was so into her that he decided to “wait” for her for two years. Then he married her and had kids with her. I nearly fell off my chair when Jerry summed up: “You’ll find a good girl. If you find one who says ‘no,’ that’s the one you want.”

The girl who says “no” is the one you pursue.

Next, Ms Thomas arose to impart formal education that would back Jerry’s anecdotes. As the principal would later tell the media, Ms Thomas wasn’t technically teaching “abstinence only”. Oh no. She said that if you had to have sex, you should use a condom. But, she warned the kids, they fail 18% of the time.

She didn’t explain that you can reduce that rate substantially by taking good care. She didn’t teach that using a backup form of birth control is a very good idea. She didn’t reveal that you could have forms of sex that wouldn’t get you pregnant. She just assumed “sex” meant penile-vaginal intercourse (everybody is straight, right?), and suggested “sex” was a pregnancy time bomb.

To prove to the kids just how high the risk supposedly is, she had them do an exercise. Each was assigned a number from one to six. She rolled a die. If your number came up, your condom failed. And pregnancy resulted. And you had to take a paper “baby”. Lesson: every sixth time a man and woman have sex with a condom, a child will result.

At this point, I tweeted: “There are apparently no scissors in the room for paper abortions.” That got a lot of favorites.

Why did I decide to live-tweet that class? I knew I had to stay quiet during it, and writing keeps my mouth shut. Moreover, I had picked up a lot of my then 2,600 Twitter followers from writing about sex ed. I thought they might be interested.

I never thought the class was going to be so awful, or that my tweets would ultimately be read by thousands of people. But waking up the next day to see my tweets on the front page of the Washington Post suggests that many people were just as shocked as I was.

The flood of communication since has also indicated that there are many men – but especially women – who have suffered from this kind of shame-and-fear-based sex ed. One wrote today to ask whether I thought she might ever be able to enjoy sex. These stories of lifelong psychological damage are so unnecessary.

Then yesterday I heard from a local teenager about a classmate who got pregnant and was so ashamed that she tried everything she could do to herself to induce a miscarriage.

The principal keeps insisting to the press that this wasn’t “abstinence-only” education. But his protests have only made me realize that “not-abstinence-only” might be the worst kind of sex ed – worse than just teaching “abstinence-only” –because it results in a written curriculum that makes parents like me foolishly assume it will be reasonable. It will cover condoms! It will talk about unwanted pregnancy!

In fact what’s getting taught is this: condoms fail constantly, sex is deeply dangerous and shameful, and “the girl you want is the girl who says ‘no’”.

We need sex ed that teaches the facts – including that sex can be pleasurable, that varied forms exist, that consent is key, and that risks should be managed wisely. If we teach that, maybe kids will come to us when they have questions or get in trouble. Maybe they won’t struggle with shame when masturbating or having sex with others.

As it is, my son has told me is that all he has really learned from this class is not to trust authority. A good lesson, but not one that he needs from the people charged with teaching him about sex.