It is not automatically wrong for a Christian to marry a non-virgin. There are several reasons why this is the case. First of all, the Bible does not say that it is wrong to marry a non-virgin.
Second, let's say that a man and a woman are married and have children and the husband dies. The woman is then free to marry another. It would not be a sin for a man to marry her in that case simply because she was not a virgin. Paul the Apostle talked about remarriage as it relates to the law. Let's take a look
"For the married woman is bound by law to her husband while he is living; but if her husband dies, she is released from the law concerning the husband. 3 So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man," (Romans 7:1-3).
Paul taught that the woman who is not a virgin and marries another after the death of her husband has done nothing wrong. She is not an adulterous because she's free from the law. Furthermore, there is nothing in the context that says it would be wrong for a man to marry her.
At one point in Scripture, we have the record of Sadducees coming to Jesus to try and trick him about multiple marriages.
"Some Sadducees (who say that there is no resurrection) came to Jesus, and began questioning Him, saying, 19 'Teacher, Moses wrote for us that IF A MAN’S BROTHER DIES and leaves behind a wife AND LEAVES NO CHILD, HIS BROTHER SHOULD MARRY THE WIFE AND RAISE UP CHILDREN TO HIS BROTHER. 20 “There were seven brothers; and the first took a wife, and died leaving no children. 21 The second one married her, and died leaving behind no children; and the third likewise; 22 and so all seven left no children. Last of all the woman died also. 23 In the resurrection, when they rise again, which one’s wife will she be? For all seven had married her.” 24 Jesus said to them, 'Is this not the reason you are mistaken, that you do not understand the Scriptures or the power of God?' 25 “For when they rise from the dead, they neither marry nor are given in marriage, but are like angels in heaven," (Mark 12:18-25).
The Sadducees, who denied that there is a resurrection, were trying to trick Jesus. But, they failed to understand the nature of the resurrection. Notice, however, that Jesus did not condemn the act of remarriage to non-virgins in his address to the Sadducees. It would seem that Jesus had no problem with it.
Priests could not marry non-virgins in the Old Testament
A priest in the Old Testament was not to marry a non-virgin.
- "They shall not take a woman who is profaned by harlotry, nor shall they take a woman divorced from her husband; for he is holy to his God," (Leviticus 21:7).
- ‘A widow, or a divorced woman, or one who is profaned by harlotry, these he may not take; but rather he is to marry a virgin of his own people, 15 so that he will not profane his offspring among his people; for I am the Lord who sanctifies him,'" (Leviticus 21:14-15).
- "And they shall not marry a widow or a divorced woman but shall take virgins from the offspring of the house of Israel, or a widow who is the widow of a priest," (Ezekiel 44:22).