Final answer:
The Jazz Age of the 1920s was the first major cultural change in the modern era to influence sexual behavior changes in the western world. Petting parties, necking parties, and the popularization of contraception played a significant role in these changes.
Step-by-step explanation:
The first major cultural change in the modern era to influence sexual behavior changes in the western world was the Jazz Age of the 1920s. During this time, as men and women pushed social and cultural boundaries, sexual mores changed and social customs grew more permissive. Petting parties or necking parties became popular on college campuses and there was a greater emphasis on the natural and pleasurable aspects of sex. The popularization of contraception and the private space that the automobile offered also contributed to changes in sexual behavior.