Starting in our teens our sexual hormones begin to rage. This is especially true of young men. Many health experts have stated men reach their sexual (peak) at 18, and women reach theirs at 28.
If this is true then it makes sense that people would have a strong interest in having sex during (their youth) and especially if they are with someone they consider to be "special".
"The average male loses his virginity at age 16.9; females average slightly older, at 17.4. And a new study shows that genetics may be a factor: inherited traits, such as impulsivity, can make a person more or less willing to have sex at an earlier age. " - Kinsey Institute
When it comes to relationships most of us (fail our way to success). If this were not the case the vast majority of us would be married to our high school sweethearts. Odds are even if someone believes they are "in love" as a young-adult there is a good chance they won't end up together forever.
Young people are going to have sex or participate in sex acts.
When you don't have a lot of responsibilities such as having a demanding job, a mortgage, car note, kids, or insurance and utility bills to pay it's easy to be preoccupied with thoughts of sexual pleasure.
Eventually everyone's "priorities" shifts. Hopefully they'll avoid the pitfalls of STDs and unplanned pregnancy long enough for wisdom and maturity to kick in and catch up with their bodies. Essentially these youths are doing what comes natural.