Let’s read Matthew 19:3-9, “The Pharisees also came to Him, testing Him, and saying to Him, ‘Is it lawful for a man to divorce his wife for just any reason?’ And He answered and said to them, ‘Have you not read that He who made them at the beginning “made them male and female,” and said, “For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh?” So then, they are no longer two but one flesh. There what God has joined together, let not man separate.’ They said to Him, ‘Why then did Moses command to give a certificate of divorce, and to put her away?’ He said to them, ‘Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, except for immorality, and marries another, commits adultery, and WHOEVER MARRIES HER WHO IS DIVORCED COMMITS ADULTERY’” (NKJV).

In this passage the Pharisees, who hated the Lord Jesus, came to TEST HIM by asking him if one could divorce his wife for any reason. The Lord reminded them that when God instituted marriage at the very beginning (see Genesis 2:21-24) He intended marriage to be for a lifetime. It is a HOLY UNION where the man and wife become ONE FLESH. In view of this God-ordained union, the Lord Jesus solemnly declared, “Therefore what God has joined together, let not man separate.” We know that man, in the ‘hardness of his heart,’ has come up with many reasons to put away his wife (or for the woman to put away her husband) and Jesus utterly condemns this. Yet He does go on to teach them that there is ONE REASON that would justify divorce; when one spouse is unfaithful and commits adultery. This act of being joined to another man/woman in sexual intercourse results in the DISSOLVING OF THE MARRIAGE BOND. The apostle Paul spoke of this UNHOLY UNION to believers in Corinth with these words, “Now the body is not for sexual immorality but for the Lord…Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a harlot? Certainly not! For ‘the two,’ He says, ‘shall become one flesh’” (1st Corinthians 6:13, 15-16). This is such a travesty of the marriage union that the ‘innocent party’ is justified in divorcing the ‘guilty party.’

Notice, it is the ‘innocent party’ who is free to divorce the ‘guilty party’ and I believe we can also infer that the innocent victim would then be free to remarry without being guilty of adultery. It is obvious from your question that your boyfriend was the ‘guilty party’ who ended the marriage bond by committing adultery and thus he is not given the liberty to remarry. This would surely be true if his ex-wife chooses not to remarry, for in that case he should repent of his sin of adultery and hope to be reconciled to her and remarry her (again, marriage in the eyes of God was intended to be for life; that is, until one of the spouses die…see Romans 7:1-3). If he would refuse to repent of his sin of adultery and marry you, you would BOTH BE GUILTY OF ADULTERY. You may be thinking, “But what if she has remarried; would he not be free to marry me then?” Perhaps, but Scripture gives us no such example or clear teaching on this, so I can’t speak with any scriptural authority on this. I will say this though, if he hasn’t been convicted of his sin of adultery and repented of his sin before God (see Psalm 51:1-4), then he is NOT one that you should desire to marry. It is possible that he could eventually be unfaithful to you without feeling any guilt at all. If he is a true believer in the Lord Jesus Christ he should feel his guilt in his act of adultery, confess it to God, and hope to be restored to fellowship with Him (see 1st John 1:9). He should also mourn over the fact that his sin brought about the dissolution of his marriage, not only because of the heartache he caused his wife, but because of the dishonor he brought upon God who ordained marriage to be permanent.  (DO)  (501.3)