Sterility or worse. Your doc may decide not to order a simple urine screening test to pickup chlamydia or gonorrhea during your visit.Since these are often asymptomatic in females, you could have them for months while they live in your fallopian tubes and destroy the linings.This could make you sterile or at high risk of a life threatening tubal pregnancy.You doc wants to do right by you, so being honest can help.
why lie? The major question is why would you lie about it? Probably nothing bad will happen but i cannot understand why you would not want to provide your doctor with an accurate medical history.