You shall not have intercourse with your neighbor's wife, to be defiled with her." (Leviticus 18:20)
The literal commandment is that a man must not have intercourse with his neighbor's wife.
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
The physical intimacy of sexual union between a man and woman (husband and wife) provides a physical picture of the spiritual intimacy that is supposed to exist between G-d and humanity and between Messiah and the bride of Messiah (the assembly of believers).
When that physical intimacy is corrupted and adulterated with relations outside the context of that marriage then the picture is defiled and the message humanity is supposed to convey becomes corrupted. In the words of this passage of Scripture, it is "defiled".
Just like there are certain animals that were not created to be eaten, there are certain people that were not created to be married (the only context in which sexual intimacy is permitted). People aren't supposed to eat pork, shellfish, cats, or dogs. Men aren't supposed to marry or have sexual relations with someone else's wife.
In a similar fashion, the Messiah is for salvation. There is no other part of creation intended for that purpose.
Messiah fulfilled this commandment by not having intercourse with his neighbor's wife.
The traditional observance of this commandment is found in its literal sense: men not having intercourse with their neighbor's wife.
The focus is not exclusively on "intercourse". Any type of pleasurable sexual contact is strictly forbidden.
We are able to fulfill this commandment today and men should not have intercourse with our neighbor's wives.
Is the focus literally on "neighbor"? What about someone who lives in another town or another state? Are they your neighbor?
Messiah addresses the question of "who is my neighbor" in the parable of the good Samaritan. Anyone we encounter in life becomes our neighbor and our "neighborly" responsibilities extend to them wherever we may be.