Complex sentence: In some sexual relationships among teenagers in the U.S., the predominant activity is not penetrative sex, but rather oral sex and mutual masturbation, as this reduces the risks associated with sexual promiscuity, such as pregnancy and sexually transmitted diseases.
Split into simpler sentences: In some sexual relationships among teenagers in the U.S., the predominant activity is not penetrative sex, but rather oral sex and mutual masturbation. Many teenagers feel that this reduces the risks associated with sexual promiscuity, such as pregnancy and sexually transmitted diseases.