The case for premarital sex being immoral really only works inside the context of religion (mostly Christianity, Islam, and Hinduism). Apart from this and personal reasons, this idea doesn't hold up well, as sex is natural, pleasurable, and not harmful if between two consenting adults. All of these characteristics defy the traditionally understood meaning of "immoral".

Even within the realm of religion there is debate over whether the text was interpreted accurately or not. For example, the Bible does not specifically mention "premarital sex", but rather refers to "sexual immorality". This has been interpreted to mean premarital sex, but this is not self evident.

While one's views on this subject largely depend on religious orientation, because it is not harmful and typically would not fit the description of secular immorality, it would be illogical to label premarital sex as universally immoral.

Further, the excessive shame surrounding premarital sex, also known as purity culture, has resulted in a number of issues for many women, including but definitely not limited to shame, sexual ignorance and intimacy issues in healthy relationships.

It's objectively immoral, each act destroys the underlying fabric of society a little bit each time, destroys the sinner's psyche a little bit each time, and inherently should be avoided, but we all do immoral things, some with far greater costs to ourselves and society. Sin can be forgiven. Evil cannot. Sex outside of marriage can be called sinful, but it cannot be called evil.