It seems to me that abstinence is unrealistic. Perhaps, more important, it doesn't encourage a young individual to think for him or herself. Sexual relations is so complicated and multilayered.
How do you treat yourself and others respectfully and with compassion? How do you give expression to your love, playfulness and hormones without hurting yourself and others? Abstinence doesn't allow one to explore all aspects of your mental, emotional, spiritual and physical self. It is a quick, though not easy answer. Way too simple. And without sex education on all its levels how can a young person look realistically at the social world around them.
Teaching only abstinence is folly.