By RoP:F p.9ff Faeries have no immortal soul, but "are spirits with bodies made of incidental matter, held together with a type of spiritual energy called glamour, and moved with stolen vitality." There are likely theologians (see ArM5 p.182 as quoted above), who don't distinguish Faerie and Infernal.
Not having an immortal - or, with Aquinas, incorruptible - soul does not make a being dead or Infernal, of course: trees and dogs are certainly alive, and by scholastic theology even have Aristotelian plant and animal souls (animae), which are corruptible.
But a human fully transformed into a Faerie should have lost her immortal soul - which makes her 'dead' in a way to any Christian community she may have belonged to, and would exclude her from the benefit of the marriage sacrament.
ArM5 hat its own, fantastic, representation of 'benandanti' as HMRE p.103ff Nightwalkers, especially p.112f Benandanti and p.113f The Hounds of God. They are humans practicing magic. The characters are meant as conventional companions (p.103) not needing the Gift.
Carlo Ginzburg's Night Battles are referenced on HMRE p.142 Nightwalker Sources.
The concept of lusting after your wife threw me for a minute. To a Cathar, I suppose you can lust after your wife, as they regard procreative intercourse a sinful act. To a Catholic priest, the bar is higher. To lust after your wife is to be so consumed by desire that you neglect other duties, probably. I don't think the priest would find the man sinful on that point.
On the point of "Repent them all; God will know his own," this is an opportunity for the priest to educate the man on basic points.
For the second case, a) if a man was married under false pretenses, there's a case for annulment. b) if the spouse somehow transformed into a magical or faerie being and left, there may be a case that the wife died. He should have a mass for her.
If the wife is still around, he's pretty much stuck as long as she seems mostly the same and nothing else forces the issue. The case seems to imply that he objects to her nature / transformation.
objects to or is insecure about. If she has transformed into a faerie and some people insist that faeries are just another form of demon, the man is likely to feel uncomfortable...
or if she is transformed into a faerie of a type that eats their spouse then his future does not look very bright, and being able to have her declared not his spouse could literally save his life... there are a lot of possibilities.