The Literal Commandment
'A widow, or a divorced woman, or one who is profaned by harlotry, these he may not take; but rather he is to marry a virgin of his own people, so that he will not profane his offspring among his people; for I am the LORD who sanctifies him.'" (Leviticus 21:15)
The literal commandment is that the high priest (verse 10) should not profane his offspring [literally: seed] among his people. Since that would occur through sexual relations with someone G-d has forbidden to him this prohibition is understood that he cannot have sexual relations (even outside of marriage) with a widow.
Messiah Says
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)
Pictures of Messiah
The high priest pictures Messiah. Since the high priest is not allowed marital relations with someone forbidden to him so, too, we know that Messiah is not allowed special relations with anyone aside from His Bride.
How Messiah Fulfilled
Messiah did not fulfill this commandment. This commandment was given to the Levites... specifically the high priest. Since Messiah was not of the tribe of Levi He did not bear responsibility to perform this commandment.
Traditional Observance
The fulfillment of this commandment was in its literal sense: the high priest did not have sexual relations with a widow either in or out of the context of marriage.
Other Notes
We can not fulfill this commandment today absent the Temple and a functioning Levitical priesthood. There is no high priest to whom this commandment would apply.
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