As the Bible say sex before marriage is a sin, is it necessary for a person who is no longer a virgin to abstain until she or he gets married?
Sin doesn't happen just the first time you do it. A person who steals is not a thief just the first time he steals. The sin remains every time he steals.
In the same way, the sin of fornication is not about maintaining virginity. It is a sin every time a person has sex with someone the person is not married to. Being a Christian is about staying away from sin. "What shall we say then? Are we to continue in sin so that grace may increase? May it never be! How shall we who died to sin still live in it?" (Romans 6:1-2).