Dufieux's Nature et virginité p. 415 notes one pre-Christian culture that respected perpetual virginity:
Dans la Germanie et dans les Gaules, il y avait des druidesses vouées à une virginité perpétuelle.
[In Germany and in the Gauls, there were druidesses vowed to perpetual virginity.]
What about their mythology made them vow themselves to perpetual virginity?
Perhaps an answer can be found from a different group of Celtic people, the Welsh: Math fab Mathonwy was a king of Gwynedd who needed to rest his feet in the lap of a virgin unless he was at war, or he would die. (Wikipedia).
In the book Celtic goddesses by Miranda J. Aldhouse-Green, pg. 70, she writes:
The sexual imagery of the goddess can also be less obvious: in the story of Da Derga's Hostel (see p.73), the doomed King Conaire refused entry to a crone, who was the Badbh in her guise as sovereignty goddess, because he was under a geis or taboo whereby he could not receive the company of a single woman after sunset. So, even if the woman was an ancient hag, there appears to have been the peril of contamination of male by female, a sexual danger which may - like the myth of Samson and Delilah - be associated with male weakness brought about by women. The converse of this may in fact be present in the Welsh Tale of Math (see Chapter 3). The concetrated, undissipated sexual energy of the virgin may be another manifestation of female sovereignty.
And indeed, it seems that the Germanic seeresses were held in high regard:
Veleda was an unmarried woman who enjoyed wide influence over the tribe of the Bructeri. The Germans traditionally regard many of the female sex as prophetic, and indeed, by an excess of superstition, as divine. This was a case in point. Veleda's prestige stood high, for she had foretold the German successes and the extermination of the legions. (Tacitus, Histories, 4.61)
With these proposals they first calmed the Tencteri and then sent a delegation to Civilis and Veleda with gifts which obtained from them everything that the people of Cologne desired; yet the embassy was not allowed to approach Veleda herself and address her directly: they were kept from seeing her to inspire them with more respect. She herself lived in a high tower; one of her relatives, chosen for the purpose, carried to her the questions and brought back her answers, as if he were the messenger of a god. (Tacitus, Histories, 4.65)
There is, in fact, no evidence of druidesses existing.
Admittedly the evidence of any kind about druids is meager, and what we have is often bad -- for instance, Julius Caesar tells us they have high position in Gaul, but then in his discussion of his conquest make no mention of any druid, which seems implausible if they were that prominent -- but it does not include female members.
This very lack of evidence has led to a lot of very ill-founded speculation. To convert Veleda -- a German seeress, and though they recorded she was unmarried, they did not record that this was connected with her position -- into a kind of order of Vestal Virgins would be a typical exaggeration, and less than some.
You may find Ronald Hutton's The Druids of interest.
It seems to be more a societal caste than something inspired by their mythology, although virginity has been associated in many cultures with the ability to prophecy.
Higgins The Celtic Druids p. 187:
The Druidesses were divided into three classes. The first were those who had vowed perpetual virginity, and lived together in communities separate from the world. Mela† gives a description of one of these nunneries. It was situated on an island in the British sea, and contained nine of them. Strange accounts are told of their magical incantations, &c. &c., just what, under the circumstances, might be expected. They are said to have pretended to possess an oracle. The second class were married, but spent their time in company with the Druids, and in the offices of religion; having conversation only occasionally with their husbands. The third class was the lowest, and were, in fact, only servants in the temples and attendants on the Druids.
St. Brigid belonged to the first class, being a Chief Druidess before her conversion to Christianity.
