The greater risk is with 'heavy petting', petting below the waist and inside the pants and underwear, and then only if the same hand touches both person's genitals meaning that when touching yourself and then your partner, you can transfer body fluids, including sperm, to your partner's genitals.
This idea encapsulates decades of fear of sex, which has its roots, for both sexes, in the church and notions of mortal sin and—for young men—perhaps a latent fear of women as well.
Eve Blue eventually gave up on petting parties she was nearly raped.
Describing pre-intercourse sexual acts, or foreplay, the word petting manages to capture both the innocence and the euphemistic repression that we identify with the s and early s.
On a more serious note, it only makes sense that since petting is progressively more sexual, that it carries greater risks and consequences.
It started with falling birth rates in the early 20th century; whereas households of 10 children were the norm for previous generations, married couples began making a conscious choice to have smaller families.