He was one of those parents who really frighten me with their lack of insight.

"The law is ridiculous, it should be changed," he said aggressively. "I have two sons, and if one of them has sex with a 15-year-old girl, I don't want him to be charged with statutory rape."

"Did it ever occur to you that the law might be there to protect your sons too, that they might not be sexually safe?" I asked.

It hadn't.

I went on to explain that condoms do not give full protection against some of the more common sexually transmitted diseases (STDS). There are over 30 of them and it is not uncommon for a person to get more than one.

Some people who are infected don't show any symptoms at all, so people who have no outward signs of being infected can pass on a nasty disease without being aware that they are harming a partner.

I went on to tell him about an exercise I use in schools to educate students about the risks they take if they decide to have sex with a person whose sexual history is unknown.

I invite two volunteers to stand up at the top of the class to represent a young couple whose sexual history we are going to trace. It is the girl's first time having sex; the guy has had one previous partner.

I invite another student up to stand to her right to represent that girl. She had two other sexual partners - one was a virgin, the other man had been sexually active with three other women.

So we bring four more students up to represent all of them. We follow on with the exercise. Within three minutes every pupil in the class is standing to the ex-virgin's right.

This is an exercise that clearly illustrates the risks involved. A young girl having sex for the first time is exposed to possible infections from her partner if he has a sexual history, and also to any infections that any person who has been sexually intimate with him might have contracted.

Everyone who has been intimate with any partner's partner must be counted. The figures can rapidly rise to 20, 30 and more. The man said nothing. He walked away without making any comment.

Parents like him frighten me because they are so unaware of the possibly fatal risks to which sexually active teenagers are exposed. I sincerely hope he went away to think things out again.

I believe we cannot stop teenagers from experimenting sexually - but I think he heard that statement from me as backing up his view that if young people ignore the law and engage in underage sex, the law needs to be changed.

If he had more facts, I wonder if he would still hold that view?

For example, it is estimated that about 10 per cent of men with gonorrhea show no symptoms, and it is one of the diseases that can also be spread orally. Chlamydia and gonorrhea often go together, and the former is particularly serious for women: it can cause scarring of the fallopian tubes and sterility.

There are drugs to treat both conditions. However it's not uncommon for young people who do not take them in the way prescribed or for the full duration of the course of treatment to get the disease again.

Genital warts are caused by the human papilloma virus. They are not always visible and can grow on the genitals, in the urethra, vagina and throat. Although they can be treated, they often recur.

Genital herpes is another nasty disease, and the belief that it is contagious only when sores are present is not accurate. Studies show that some people spread the disease even when they have no sores.

Condoms do not protect men from herpes, as vaginal secretions can get on areas of the body not protected by the sheath. The symptoms can be treated, but there is no cure.

I regret that I didn't get the opportunity to tell that man that all it would take is one time with one infected partner for his son to contract a sexually transmitted virus that will be with him for life.

The law is there to safeguard young people of both sexes from the risks attached to premature sexual activity. They certainly need that protection.