Laudator Temporis Acti

James Joyce (1882-1941), A Portrait of the Artist as a Young Man. Introduction and Notes by Dr Jacqueline Belanger (Hertfordshire: Wordsworth, 2001), p. 182 (from chapter 5), with note on p. 235:

Hell, Temple said. I can respect that invention of the grey spouse of Satan.⁴⁷⁷

477 (p. 182) grey spouse of Satan 'Sin' is named as Satan's daughter and wife in Book II of Milton's Paradise Lost, and their son is Death.

Don Gifford, Joyce Annotated: Notes for Dubliners and A Portrait of the Artist as a Young Man, 2nd ed. (Berkeley: University of California Press, 1982), p. 274:

grey spouse of Satan In Milton's Paradise Lost, book II, Satan encounters his daughter-wife Sin and the son of their incest, Death, at the gates of Hell; she is not described as "grey" but as "fair" above and "foul" below.

It isn't to Milton's Paradise Lost that we should look for an explanation of the phrase "grey spouse of Satan," but to Swinburne's poem "The Monument of Giordano Bruno," lines 15-24, esp. line 16:

Cover thine eyes and weep, O child of hell,        15
    Grey spouse of Satan, Church of name abhorred.
    Weep, withered harlot, with thy weeping lord,
Now none will buy the heaven thou hast to sell
At price of prostituted souls, and swell
    Thy loveless list of lovers. Fire and sword        20
    No more are thine: the steel, the wheel, the cord,
The flames that rose round living limbs, and fell
In lifeless ash and ember, now no more
    Approve thee godlike.

To both Swinburne and Joyce, the "grey spouse of Satan" was the Catholic Church.