They say the seeds for this transformation lay in the High Middle Ages:
[I]n the Middle Ages, female mystics, following the lead of Catholic thinkers like Bernard of Clairvaux, began developing an interpretation of the bridegroom/bride relationship as representing that which existed not only between Christ and the collective church, but Christ and the individual soul. Jesus became not only a global savior, but a personal lover, whose union with believers was described by Christian mystics with erotic imagery. Drawing on the Old Testament’s Song of Songs, but again, using it as an allegory to describe God’s relationship with an individual, rather than with his entire people (as it had traditionally been interpreted), they developed a new way for the Christian to relate to Christ – one marked by intimate longing.I know quite a bit about the expressions of Christianity in the High Middle Ages, and I'd have to say that this was at that time very much an undercurrent of the faith. The roots may lay there, but that isn't how you see the faith being portrayed in either the scholarly writing of churchmen, or else the popular songs and tales of the era. It could be that they are right that this undercurrent informed a major shift in later periods, of course.
For example, the German nun Margareta Ebna (1291-1351) described Jesus as piercing her “with a swift shot from His spear of love” and exulted in feeling his “wondrous powerful thrusts against my heart,” though she complained that “[s]ometimes I could not endure it when the strong thrusts came against me for they harmed my insides so that I became greatly swollen like a woman great with child.”
The idea of Christian-as-Bride-of-Christ would migrate from Catholicism to Protestantism, and be picked up even by the dour Puritans who journeyed to American shores. Mather himself declared that “Our SAVIOR does Marry Himself unto the Church in general, But He does also Marry Himself to every Individual Believer.” Mather’s fellow Puritan leader, Thomas Hooker, preached that:“Every true believer . . . is so joined unto the Lord, that he becomes one spirit; as the adulterer and the adultresse is one flesh. . . . That which makes the love of a husband increase toward his wife is this, Hee is satisfied with her breasts at all times, and then hee comes to be ravished with her love . . . so the will chuseth Christ, and it is fully satisfied with him. . . . I say this is a total union, the whole nature of the Saviour, and the whole nature of a believer are knit together; the bond of matrimony knits these two together, . . . we feed upon Christ, and grow upon Christ, and are married to Christ.”That which was present at the founding of the country, grew to become part and parcel of American Christianity, especially its evangelical strain, and continues to play a significant role in influencing the language and ethos of the faith today.
In Why Men Hate Going to Church, David Murrow points to examples of how the bridal imagery born in the Middle Ages continues into the modern age, citing books with titles like Falling in Love With Jesus: Abandoning Yourself to the Greatest Romance of Your Life, and authors who “vigorously encourage women to imagine Jesus as their personal lover”:“One tells her readers to ‘develop an affair with the one and only Lover who will truly satisfy your innermost desires: Jesus Christ.’While much of what Murrow calls “Jesus-is-my-boyfriend imagery” is directed at women, Murrow believes it has become suffused throughout the entire faith, and “migrate[d] to men as well.” “These days,” he writes, “it’s fairly common for pastors to describe a devout male as being ‘totally in love with Jesus.’ I’ve heard more than one men’s minister imploring a crowd of guys to fall deeply in love with the Savior.’”
Another offers this breathless description of God’s love: ‘This Someone entered your world and revealed to you that He is your true Husband. Then He dressed you in a wedding gown whiter than the whitest linen. You felt virginal again. And alive! He kissed you with grace and vowed never to leave you or forsake you. And you longed to go and be with Him.’”
Still, in the Middle Ages, the eroticism of the faith is as likely to provide an erotic attraction for men as well as for women. As late as The Faerie Queene, the church-as-bride metaphor was being employed in a kind of dual way: Una symbolizes the church that is the bride of Christ, but Una herself weds to St. George as his reward for his dragonslaying virtue. The Grail Maiden, daughter of King Pelles, ends up seducing Lancelot (right after he too slays a dragon; Elaine only later marries him) and being the mother of Galahad (who fulfills the Grail quest through chastity, not eroticism). Lancelot is chosen by God to father Galahad on Elaine, according to the story, because he (like St. George) is a living flower of valorous knighthood. Service to God and erotic success are linked for men in these stories. Living out the noble virtues honored by the faith makes one worthy of love and beauty, which will be put by God to further service through the stability of one's marriage and the fates of one's children.
Thus, if the Bride of Christ narrative was as important for female mystics as the authors imply, that only creates parity, not a "feminization" of Christianity in the High Middle Ages. One might, of course, question whether or not eroticism is really even appropriate as a religious motivation; but whether it is or is not, it certainly was not an exclusively female (let alone 'feminizing') one. It was a regular justification for the most manly of the virtues (though note that, in The Faerie Queene, there is a female knight exercising these mostly-manly virtues; and there is at least one in the older Arthurian corpus as well).
As for their later periods, I am less well placed to provide a useful critique.