The real question is how may a relationship between unmarried people can be infused with k'dushah, holiness. This is also a question between marrieds, but at least in that situation there are more models. And this question does not just apply to sexual relationships. We might well devote attention to how one becomes a holy friend, a holy companion, before we ask about becoming a holy lover. The issue of premarital sex is an issue for adolescents.
Adolescence is the period between physical maturity and socio-economic maturity.
As others have pointed out here, in our era, physical maturity comes earlier than it ever has before, probably due to better childhood nutrition. And socio-economic maturity come later than ever before, due to the length of time it now takes to establish oneself in an adult career. Nonetheless, nobody expected them to remain celibate. Instead, the community encouraged them, and just about everybody else, to marry shortly after attaining physical maturity, even though they might have to be supported by one or the other set of parents for several years afterward, or even indefinitely. After WWII, the GI Bill enabled a whole generation of veterans to complete college and sometimes graduate school and establish themselves in adult careers while being husbands and fathers in families established just before or just after military service.