The Atheist Perspective
What has to be justified is the claim that it's not moral to have sex before marriage. That it is moral has no more need of being justified than the claim that eating ice cream or dancing or skydiving is moral.
Sex is not in any relevant way objectively different from other human activities that involve people in each other's lives. As such, sex is always subject to moral rules (honesty, compassion, responsibility). But none of them entail any need to be married.
Just treat your sexual partners considerately, responsibly, and honestly.