She was heckled by anti-abortion protesters as she arrived, and was interrupted in mid-speech by two women who called her phoney and said Marie Stopes was interfering with their country in Latin America.
The singer made her frank comments as she addressed a United Nations conference in London, where she also spoke of the pressure she felt to lose her virginity when she was younger.
Halliwell called for sex education for every person in the world and told the conference that 600,000 women died every year as a result of pregnancy.
"That's 22 double decker buses full of pregnant women crashing to their death every day - no survivors," she said.
"And this is the year 2000. We're not living in Victorian times."
The former Spice Girl and UN Goodwill Ambassador joined peacekeeper Lieutenant Fintan McCarthy to launch the event, the Model United Nations Millennium Summit, on Saturday.
She was heckled by anti-abortion protesters as she arrived, and was interrupted in mid-speech by two women who called her phoney and said Marie Stopes was interfering with their country in Latin America.
They were herded out by security men - and Halliwell told her audience: "Everyone's allowed to have their opinion - I applaud that."
More than 120 young people aged 16 to 18 travelled to the two-day event where they will wrangle with the great issues facing the world today.
Halliwell opened the summit, at the International Maritime Organisation Headquarters, with a speech covering her work with the UN Population Fund and Marie Stopes International.
She told them: "We either find sex embarrassing or we think we know it all - maybe you do, but I know I certainly don't.
"In fact I think puberty is really hard, and dealing with the peer pressure to become sexually active is even harder.
"I remember the massive pressure to lose one's virginity - everyone else seemed to have done it."
Lt McCarthy, who recently served as platoon commander with the UN peacekeeping force in the Lebanon, is also set to deliver an address.
Next generation
Video greetings were played at the opening ceremony by Prime Minister Tony Blair and UN Secretary-General Kofi Annan, whose message reads: "You represent the next generation of world citizens, and you are telling us that all nations must work together to make the 21st Century better than the 20th."
At the end of this weekend's summit, the participating students will produce a MUNSUM Declaration incorporating their conclusions on the global issues facing the UN in the new millennium.
It will be presented to the real world leaders at the UN Headquarters in New York during the UN Summit in September.
Geri Halliwell became a UN Goodwill Ambassador in 1998, following in the footsteps of stars such as Audrey Hepburn and Sophia Loren.
She visited the Philippines last year on behalf of the UN Population Fund where she promoted safe sex and contraception to young people, drawing criticism from the Catholic church.