Answer: It is neither always wrong nor always right to hold off having sex until one is married. Whether it is right or wrong for you is a personal decision you have to make.
Here are some Objectivist principles one could benefit from if considering the issue:
One should always pursue happiness in the full context of one's life. Live in the moment, but live for the long-term.
Romantic love is the most intimate relation one can have with another person. Real romantic love arises from an appreciation of the totality of the loved one: their character, talents, beauty, presence, achievements and so on.
Sex is a profound expression of this total appreciation, for in it one loves and is loved in body as well as person.
To have sex with another person without a profound personal connection is to miss out on a key part of what sex can and should be.
To deny oneself sex out of a principle of self-denial is to miss out on a key aspect of romantic love.
Marriage is in most places a complex contract with many implications, defined and enforced by the State. It is not necessarily the case that this contract is the best way to solemnize a serious, long-term romantic commitment.
favors a rational view of the world and the pursuit of happiness in life. So it opposes many common rationales for pre-marital chastity. For example, some people regard sex outside of marriage as blasphemous. This is at best a metaphor for a rough rule of thumb, and it is false on its face. Some people believe the pleasure of the body is sinful. Objectivism
rejects this view, and advocates mind-body unity and life-affirming pleasures of all sorts.
In any case, the decision is up to each person. People vary in the degree to which they can enjoy relatively casual sex. People vary in the degree to which they enjoy sex at all. People vary in the degree to which they would enjoy having only one sexual partner over the course of their lives.
The Objectivist principles I’ve mentioned indicate the following: denying oneself the pleasure of sex for the sake of chastity is wrong. So one should wait for marriage only if one’s marriage will coincide with being in love with a worthy person. The important thing is to enjoy the pleasure of sex with someone who is worthy, and when it will make you happy.