It's the first cite I found, but if that's not religion-neutral enough for you, I can find plenty more.
Sex causes the release of hormones such as oxytocin. Most people aren't capable, especially when young, of having sex with someone, never seeing that person again, and being okay with the experience.
"Even when romantic love is not linked with reproduction, this passion can provide a teen or young adult with some exceptional personal and social benefits, including exhilarating joy, increased energy and optimism, feelings of intimacy, self-esteem, inclusion in health-giving social groups, exercise, social and personal support, and crucial practice in the skills of building a long-term partnership—skills they will need to make the most important social contract of their reproductive lives."