The purpose of Matthew 19:9 was the protection of the marriage institution. Sandwiched between a discussion of the permanence of marriage using the “one flesh” words from Genesis 2:24 and a discussion about eunuchs, the passage is rather obviously about physical acts of fornication on the part of a spouse, that give the only reason accepted by God for exiting a marriage and entering another. When we say that the viewing of pornography is porneia–that it constitutes adultery– thus allowing for the spouse of the viewer to be free from the marriage bond and remarry another, we cheapen the institution God was protecting in the passage. Further, if the temporal consequence of lust-in-the-heart is freedom to dissolve my marriage, then the temporal consequence of hatred in my heart is the same consequence that’s due to a literal murderer (same context as “whoso looks on a woman to lust” in Matthew 5 and then in 1 John 3:15). In other words, if the pairing of Matthew 5:28 and Matthew 19:9 give license for a spouse who is guilty of lust to be divorced for adultery, then the pairing of Matthew 5:21,22 and 1 John 3:15, surely would subject the man with a heart of hatred to the same consequence as if he were a literal murderer.
The big problem in both of these scenarios is the ambiguity of the thought processes described. When does one cross the line into hatred? Can any woman reading say she is absolutely sure she has not ever hated anyone? Would I then say I might be a murderer, but I’m not sure. It’s easy for us to see that a literal murderer is not the same in every respect as one who has committed the sin of actually killing someone. in a similar way, the lust of the heart is a sin having various degrees. Which man could say he is certain that he has never had a lustful thought about anyone?
If “sexual immorality” in Matthew 19:9 indeed does include any act of immorality that is of a sexual nature (not just physical sexual activity), I would, as a woman, need to be very careful to be able to assert that I had never stepped outside the boundaries of purity in my dress, my language, my overtures, etc…for, if I have erred in any of these ways, my husband could certainly put me away, for those “acts of immorality” that are most definitely in the arena of sexuality.
God put Matthew 19:9 in our Bibles to protect our marriages. If Matthew 19:9 is an effective door of exit for anyone whose husband has ever committed the sin of lust, then may the most conscientious of our godly men, be at risk of losing their marriages because of some lingering glance or lust of the heart? Could the homes of our congregations be destroyed in wholesale fashion, without displeasing God in the “putting away” of husbands who have, at one point or another, been guilty of impure thoughts or momentary lust of the heart?
This argument that Matthew 19:9 and Matthew 5:28 make for an acceptable (to God) reason for divorce and remarriage is fallacious and extremely destructive to our families and the kingdom. “Sexual immorality” is an unfortunate translation of “porneia”.
I recommend this article for your consideration. It is deeply rooted in Scripture and in wisdom from above that is pure and peaceable (James 3:17).
Pornography is an awful sin and it is doing just about as much damage as any tool the devil has at hand to destroy our next elders, our marriages, our innocent sons and our congregations. It must not be ignored because it is a spiritual malignancy that quickly grows to stage 4. But it is not the porneia of Matthew 19:9.