Luke 16:18 is interesting because here Jesus only condemns actions of men as adultery:
"Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery."
The word "adultery" comes from the same root as "adulterate" which literally means "to render (something) poorer in quality by adding another substance, typically an inferior one". In origin, adultery is sexual behavior that corrupts the line of inheritance, and therefore can only be committed against a husband -- there's never any doubt about the identity of a child's mother, so a wife's line of inheritance cannot be corrupted. In this legal sense, the crime of adultery was not so much about morals as it was about protecting a husband's assurance of legitimate offspring. (An assurance that a wife has thanks to biology.)
Which raises the question: in the two scenarios Jesus speaks of, who is the victim of adultery? Both cases are interesting in their own right.
Case 1: "Everyone who divorces his wife and marries another commits adultery". In the literal sense the man can only be committing adultery if the woman he marries was already the wife of another man, in which case the adultery is being committed against the other man. However, Jesus doesn't directly say that the new wife is or was married, which leaves open the door to the thought that Jesus is declaring that the spurned wife is actually a victim of adultery herself.
Case 2: "[H]e who marries a woman divorced from her husband commits adultery". In this case it seems clear that the the first man is committing adultery against the divorced husband. However, if the marriage no longer exists then how can there be adultery? Perhaps the timing or circumstance of the woman's second marriage calls into question the legitimacy of the first husband's children? That seems like an overly specific reading for which there is no direct evidence.
Anyway, it's interesting to me that we've expanded the definition of "adultery" to include all sorts of marital sexual infidelity while at the same time "adulterating" the original purpose of the term: to protect husbands' assurance of paternity.
