The Vision of Dr. Faustus is a play on the old legend of a German alchemist who found a way to conjure up the devil. Upon meeting Satan, Faustus makes a pact with him. For a life of earthly pleasures Faust will sell his soul. The story of Dr. Faust has thus come to mean a scientific or magical man selling his soul for a life of decadence and pleasure.
You could consider Dunstan's peculiarly Canadian, Scottish puritanism, which created, to say the least, a conflicted idea in him about sex. For Dunstan to attain a sexual relationship was far-fetched in his mind to begin with; he was horribly mangled by the bomb explosion in the war, and had lost a leg. His early infatuation for Leola Cruikshank had fizzled to a point where he no longer wanted her, and, though he made more of a show of romantic disappointment than he actually felt, she married one of his old friends, Boy Staunton. Dunstan had been engaged to Diana in England and avoided marrying her. So Dunstan seems to stop himself from a full-fledged sexual relationship many times. Why? Is it that Dunstan just can't commit, or is it because he is hung up on the skewed (although, he believes, saintly) sexuality of the insane Mary Dempster? How does this relate to the Vision of Dr. Faustus?
I think one answer is that Dunstan feels that he is cut off from the normal course of life (such as marrying, or having a long-term, continuous relationship) because he is "twice born". In this he is like Dr. Faustus, who was given a new life after a traumatic event. Faustus sells his soul to evil, but Dunstan feels he was saved by the vision of the Virgin Mary after his near-death from the bomb in the war. Dr. Faustus was no longer part of the human race after he sold his soul -- he had magical powers to grant himself his earthly desires, and had to surrender his soul in the end. Dunstan is similar in that he had to live a life of writing hagiography (the lives of saints) to "pay back" the saint who saved his life. It's a contradition, somewhat; Dunstan owes his life to goodness, and Faust owes his life to evil -- but Dunstan was trained just enough in the puritanical thinking of early 20th-century Canadian Calvinists to believe in that sort of bargain. Dunstan had to "pay" for his saved life somehow. Maybe he did it by sacrificing any possibility of a life-long partner.
You could also consider Dunstan's fascination with Faustina, the semi-literate beauty hired by Eisengrim to adorn his illusion. Why would a man past fifty, with a less than charismatic appearance and a withered arm and a wooden leg, assume that a South American girl of less than twenty would love him? This was a kind of martyrdom. Dunstan sacrificed his love on the altar of Faustina, who would, predictably, care not a whit for him. This speaks to a certain dislike for himself, and a misunderstanding of what would be good and fulfilling for him at this time of his life. Dunstan might have thought of it, unconsciously, as penance (as he did of many other things, such as keeping the stone that hit Mary Dempster for more than 40 years.)
This is a knotty psychological problem, and I think there is more than one answer to it. Consider Dunstan's relative failure at sexual relationships, and how it would relate to the Faust legend. Also consider that this is an "illusion" -- how much of Dunstan's sexual life was an illusion, too?