Well, this is a topic which definitely hits close to home for me...and for more than one reason.
For starters, I was married for almost 18 years, and my ex ultimately committed adultery several times, and then divorced me.
With such being the case, am I the so-called "innocent party" who is free to remarry?
Honestly, I don't know, and I've read the entire Bible from cover to cover many times, and prayed about this innumerable times as well.
When it comes to the topic of marriage or anything related to it, I think that it's best that we begin by realizing that marriage was instituted by God to mirror the relationship between Christ and the church.
To this end, Paul wrote:
Ephesians chapter 5
[22] Wives, submit yourselves unto your own husbands, as unto the Lord.
[23] For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body.
[24] Therefore as the church is subject unto Christ, so let the wives be to their own husbands in every thing.
[25] Husbands, love your wives, even as Christ also loved the church, and gave himself for it;
[26] That he might sanctify and cleanse it with the washing of water by the word,
[27] That he might present it to himself a glorious church, not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish.
[28] So ought men to love their wives as their own bodies. He that loveth his wife loveth himself.
[29] For no man ever yet hated his own flesh; but nourisheth and cherisheth it, even as the Lord the church:
[30] For we are members of his body, of his flesh, and of his bones.
[31] For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.
[32] This is a great mystery: but I speak concerning Christ and the church.
[33] Nevertheless let every one of you in particular so love his wife even as himself; and the wife see that she reverence her husband.
With such in mind, I think that a good question to ponder is this:
Would Christ ever divorce someone?
In my understanding of scripture, the answer to this question is "yes".
We read:
Jeremiah chapter 3
[1] They say, If a man put away his wife, and she go from him, and become another man's, shall he return unto her again? shall not that land be greatly polluted? but thou hast played the harlot with many lovers; yet return again to me, saith the LORD.
[2] Lift up thine eyes unto the high places, and see where thou hast not been lien with. In the ways hast thou sat for them, as the Arabian in the wilderness; and thou hast polluted the land with thy whoredoms and with thy wickedness.
[3] Therefore the showers have been withholden, and there hath been no latter rain; and thou hadst a whore's forehead, thou refusedst to be ashamed.
[4] Wilt thou not from this time cry unto me, My father, thou art the guide of my youth?
[5] Will he reserve his anger for ever? will he keep it to the end? Behold, thou hast spoken and done evil things as thou couldest.
[6] The LORD said also unto me in the days of Josiah the king, Hast thou seen that which backsliding Israel hath done? she is gone up upon every high mountain and under every green tree, and there hath played the harlot.
[7] And I said after she had done all these things, Turn thou unto me. But she returned not. And her treacherous sister Judah saw it.
[8] And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce; yet her treacherous sister Judah feared not, but went and played the harlot also.
[9] And it came to pass through the lightness of her whoredom, that she defiled the land, and committed adultery with stones and with stocks.
Because of Israel's repeated spiritual whoredoms, God "put her away, and gave her a bill of divorce" (vs. 8).
Seeing how God divorced Israel for her whoredoms, I do believe, in accordance with the "exception clause", that a man can justifiably put away or divorce his wife for the cause of sexual immorality.
That said, we must also strongly consider something else that God said here, namely this:
"They say, If a man put away his wife, and she go from him, and become another man's, shall he return unto her again? shall not that land be greatly polluted? but thou hast played the harlot with many lovers; yet return again to me, saith the LORD."
While citing the famous/infamous passage from Deuteronomy chapter 24, the LORD said unto his whorish wife whom he had put away and given a bill of divorce "yet return again to me, saith the LORD".
In other words, the LORD, apparently, left the door open for reconciliation even after his divorce.
I don't claim to have all of the answers, but this definitely opens up another question for me:
If a man puts away his wife justifiably due to sexual immorality, is he then responsible to also leave open the door for reconciliation with his former spouse?
If, instead, he gets remarried, then how would this type of reconciliation which the LORD himself offered to his divorced spouse be possible?
Like I said, I don't have all of the answers, but this particular question has kept me in a holding pattern of sorts or hindered me from 100% considering the possibility of remarriage in my own life.
Beyond this, this also hits very close to home in that I'm constantly confronted with other professing Christians who are in similar situations.
In fact, just last night I was on the phone with a divorced Christian, and we were discussing this very topic.