They were controlling for age and sex, two possibly confounding variables. This is the wrong lesson. Many people quit smoking because they are sick, so the group of ex-smokers includes a lot of unhealthy people. It is not surprising that recent ex-smokers are less healthy than smokers. This is an observational study.
Women choose their own means of contraception.
The investigators were attempting to control the affects of these potentially confounding variables. Patterns of sexual activity, e.g. number of partners, vary considerably by age, education, and marital status. The conclusions drawn by the investigators were not justified. Because sexual activity and number of partners increase the risk of cervical cancer, one would expect women using the pill to have a higher rate of cervical cancer. There is no reason to conclude that the pill was causing cervical cancer.