Assalamoalaikum!!
I want to know about,sex before marriage and is this necessary to get marriage with that person with one had a sex.

All respect to Zakir Naik and I do wish they answered here, many people come asking for him, But he does not answer question here.

I am sure other good experts will come shortly to help you

Walaikumaslaam warahmatullah
Sex or any kind of private relationship before marriage is not allowed in Islam.

If both partners repent to Allaah then they are permitted to marry one another.