She said this was omitted from the range of sex education materials produced in New Zealand that she had viewed.
Dr Grossman said sex education, like other public-health initiatives, should present the "ideal", such as in nutrition.
In sexuality - "and I'm not talking about morality, I'm a physician" - the ideal was one sexual partnership for life, delayed until adulthood.
"People that are able to achieve that - not that this is so easy - never have to worry about these myriad health issues."
Health issues included STIs and human papilloma virus (HPV), which is a key factor in cervical cancers.
Dr Grossman cited a Family Planning website, theword.org.nz, and pamphlet, both aimed at young people, which encouraged those considering becoming sexually active to make sure they were ready and to talk with their partner about contraception, but did not tell them to talk to a responsible adult.
And she said sex education materials in New Zealand dangerously overstated the degree of protection people could expect from condoms, even when correctly used.
Family Planning chief executive Jackie Edmond said, "We support telling people to have loving, caring relationships and young people to delay having sexual onset.
"We don't aspire to talking about the ideal of one sexual relationship. We are pragmatic - and we are dealing with young people."
Family Planning referred to condoms as providing "safer" sex, not "safe" sex.
"You can get HPV through genital contact; condoms don't necessarily protect against that."