The sexual behavior of teenagers who pledge abstinence does not differ from that of closely matched non-pledgers. This means that those who vow to wait until marriage often do not fulfill that pledge - and are less likely to protect themselves from pregnancy and disease.
Rosenbaum, J. (2009). Patient Teenagers? A Comparison of the Sexual Behavior of Virginity Pledgers and Matched Nonpledgers PEDIATRICS, 123 (1) DOI: 10.1542/peds.2008-0407