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How then can [the priests] be both husband and wife?

My understanding is that a priest is in the person of Christ to his parish, and only in his priestly function, and at the same time he is a member of the Catholic Church as the body of believers, which is the bridegroom of Christ. This may be an impossible arrangement in a carnal marriage but we are talking a mystical marriage here. Perhpas Jo Kus can elucidate this point further.

If celibacy was a requirement as it is in some churches, it would have been explicitly set out [in the pastoral letters]

It was not a requirement at the time the Epistles were written. You really need to read the article I linked in 7552, The biblical foundation of priestly celibacy, in particular:

Scholars generally agree that the obligation of celibacy, or at least of continence, became canon law from the fourth century onwards. Here certain incontrovertible texts are quoted repeatedly: three pontifical decretals around AD 385 (Decreta and Cum in unum of Pope Siricius and Dominus inter of Siricius or Damasus) and a canon of the Council of Carthage of AD 390.11
However, it is important to observe that the legislators of the fourth and fifth centuries affirmed that this canonical enactment was based on an apostolic tradition. The Council of Carthage, for instance, said that it was fitting that those who were at the service of the divine sacraments be perfectly continent (continentes esse in omnibus): «so that what the apostles taught and antiquity itself maintained, we too may observe».12 The decree on the obligation of continence was then passed unanimously: «It is pleasing to all that bishop, priest and deacon, the guardians of purity, abstain from marital relations with their wives (ab uxori bus se abstineant) so that the perfect purity may be safeguarded of those who serve the altar.»

Paul states explicitly he has the power, the free will to marry if he chose to

Actually it is doubtful that he does. The verses in question read

4 Have not we power to eat and to drink? 5 Have we not power to carry about a woman, a sister, as well as the rest of the apostles, and the brethren of the Lord, and Cephas?
(1 Corinthians 9)

The Douay commentary is

5 "A woman, a sister"... Some erroneous translators have corrupted this text by rendering it, a sister, a wife: whereas, it is certain, St. Paul had no wife (chap. 7 ver. 7, 8) and that he only speaks of such devout women, as, according to the custom of the Jewish nation, waited upon the preachers of the gospel, and supplied them with necessaries.

At any rate, the passage clearly (cf verse 4) speaks of physical power, not any ecclesial or legal privilege.

It is of course true that the celibacy is a discipline (as opposed to an immovable dogma) of the Latin Church alone and even then it is relaxed in some circumstances. It would be wrong to confuse the carnal marriage with the mystical marriage to the Church (or, perhaps, a parish) in which a priest finds himself, and Orthodox as well as Eastern priests can marry, or, more precisely, married man can be ordained as priests in these churches. However, our understanding of the "unius uxoris vir" is incomplete without examining both the historical and the mystical aspects of the exhortation.