Having sex before marriage is the best choice for nearly everyone.
How do I know? Well, first of all, nearly everyone has sex before marriage – 95% of Americans don't wait until their wedding night. And that's a longstanding American value. Even among folks in my grandparents' generation, nine out of ten of them had sex before they wed.
Of course, just because lots of people do a thing doesn't mean it's a good thing. But sex is. In terms of happiness, sex is better than money, and having sex once a week instead of once a month is the "happiness equivalent" of an extra $50,000 a year. People with active sex lives live longer. Sex releases stress, boosts immunities, helps you sleep and is heart-healthy.
Sex is good whether you're married or not, and certainly folks who wait until marriage can have a lot of sex once they tie the knot. But waiting until marriage often means both early marriage and conservative views on marriage and gender – and people who marry early and/or hold traditional views on marriage and gender tend to have higher divorce rates and unhappier marriages.
~ from The Moral Case for Sex Before Marriage by Jill Filipovic ~
Part of "growing up" is experiencing different things. When we are young, most adults encourage us to date or get to know a wide variety of people. The thinking here is that, in order to "know" which partners may be right for you, you need a sample of more than one!
From my perspective, this general rule applies to sexual relationships just as much as any other type of relationship.
If you have only been intimate with one person, then how can you figure out if you're having good sex or not? It would be like judging the taste of all vegetables when the only vegetable you'd ever eaten is turnips! Hey, you might love turnips, but you might love parsnips or radishes even more.