In November of 2005, the Family Research Council issued an Executive Summary titled "Abstinence Until Marriage: The Best Message for Teens," by Bridget E. Maher. Here is an excerpt, quoted from the section titled Parental Influence on Teen Sex
"What influences teens to have sex? There are many factors, including sexually explicit messages in the media, peer pressure, alcohol and drugs. However, parents also play a major role in whether or not their teen will have sex. In a 2001 poll, 49 per cent of teenagers said their parents influenced their decisions about sex most strongly. "
"Parents affect teens' sexual decision making by their marital status, attitudes, supervision, and involvement in their children's lives. A 2000 study of national data found that adolescents living with both of their parents were less likely to engage in sexual activity than those living with single parents.  Also, a 1994 study found that teens living in a single parent home at age 14 were more likely to engage in sexual activity and to have sexual intercourse more frequently. "
"Parental attitudes toward sex also greatly affect teen sexual behavior. A 1996 study of black adolescents ages 14 to 17 revealed that those who believed their mothers disapproved of adolescent sexual behavior either abstained from sex or had sexual intercourse less frequently. The authors of the study emphasized that 'parents need to be firm in their emphasis on abstinence if they wish to discourage their teenage son or daughter from engaging in sexual intercourse.'  The Adolescent Health Study also confirmed the importance of maternal disapproval of sex. Among 8th-11th graders, 'when teens perceive that their mother strongly disapproves of them having sex, they are more likely to delay initial sexual intercourse.' "
"Parental supervision and emotional connectedness between parents and teens play a big role in whether or not teens engage in sexual activity. A 1994 study found that teens living in neighborhoods where most parents work full time and provide less supervision were more likely to have sex.  Studies have also confirmed the importance of parent-child connectedness. A 1997 longitudinal health study of 12,000 adolescents found that teens were more likely to delay intercourse when they felt emotionally connected to their parents and when their parents disapproved of their being sexually active or of using contraception.  The Adolescent Health Study also found that 'high levels of mother-child connectedness are independently related to delays in first sexual intercourse among 8th and 9th grade boys and girls and among 10th and 11th grade boys.' "
International Communications Research for National Campaign to Prevent Teen Pregnancy, as cited in "Faithful Nation: What American Adults and Teens Think About Faith, Morals, Religion and Teen Pregnancy," National Campaign to Prevent Teen Pregnancy, September, 2001.
19 John S. Santelli et al, "The Association of Sexual Behaviors with Socioeconomic Status, Family Structure and Race/Ethnicity Among U.S. Adolescents," American Journal of Public Health 90 (October 200): 1582-1588.
20 John O.G. Billy et al, "Contextual Effects of the Sexual Behavior of Adolescent Women," Journal of Marriage and the Family 56 (May, 1994): 387-404.
21 James Jaccard, Patricia J. Dittus and Vivian V. Gordon, "Maternal Correlates of Adolescent Sexual and Contraceptive Behavior," Family Planning Perspectives 28 (July/August 1996): 159-165, 185.
22 Robert W. Blum, "Mothers' Influence on Teen Sex: Connections That Promote Postponing Sexual Intercourse," Center for Adolescent Health and Development, University of Minnesota, 2002, 16.
23 John O.G. Billy et al, "Contextual Effects of the Sexual Behavior of Adolescent Women"
24 Michael D. Resnick et al, "Protecting Adolescents from Harm: Findings from the National Longitudinal Study on Adolescent Health," Journal of American Medicine 278 (September 10, 1997): 823-832.
25 Robert W. Blum, "Mothers' Influence on Teen Sex: Connections That Promote Postponing Sexual Intercourse," 18.