The idea behind this statement is that when one has reached emotional, personal, cultural, cognitive and social maturation there exists the ability to recognize that there are consequences for one's actions and to act accordingly--what may be called "acting responsibly". Additionally, with maturation there is a social and cultural expectation that one thinks of how his/her actions will affect the self both short-term (immediately) and long-term (in the future) as well as how one's actions will affect others.
Conversely, when one lacks emotional and social maturation one is less able to think and act in ways that fully consider the short-term and long-term consequences to self or to others. Acting with disregard to consequences to self and others can be said to be "irresponsible."
Applying these definitions to adolescent sexuality: adolescents are by definition still undergoing the maturation process. Social and cultural influences can alter this maturation period, but some researchers have argued that the brain does not become fully developed maturationally speaking until around the age of 23.
If we accept these premises, then adolescents are incapable of having fully mature thought processes under many ciircumstances--including considerations about the consequences of sexual behavior. Such consequences may include pregnancy, STDs, adverse social reactions such as judgment, ridicule or alienation from one's social circle or family, feelings of confusion, guilt or remorse, unrealistic or inappropriate expectations about one's sexual partner, etc.
In short, because adolescents are lacking full cognitive maturity they are ill-equipped to make good judgments about important behaviors like sexual activity. In other words,adolescents are far less able to fully grasp the short-term and long-term consequences of how their sexual behavior may affect themselves and others.
In summary, applying the statement to sexual behavior: adolescents are still emotionally, cognitively and socially immature and therefore more likely to make irresponsible choices such as not considering how their sexual behavior may affect themselves and others personally, culturally, spiritually, financially, and socially, both in the immediate and long-term future.