Exposition of the Gospel of Luke 2.6–7 — as quoted by Pitre (p 115)
Fittingly is [Mary] espoused, but Virgin because she prefigures the Church which is undefiled yet wed. A Virgin conceived us of the Spirit, a Virgin brings us forth without travail.
No freely usable English translation of this passage is online, so the note keeps Pitre's quotation and links to the full text. Translation: as quoted by Pitre (via Gambero); the Luke commentary is not in the NPNF selection.
In Jewish Roots of Mary
Why it is cited
Mary, espoused yet virgin, 'prefigures the Church which is undefiled yet wed. A Virgin conceived us of the Spirit, a Virgin brings us forth without travail' — Mary as icon of the Church, and Pitre's bridge to the painless birth.