The phrase as demure as a whore at a christening, and its variants, mean having an appearance of respectability.

The image of a prostitute at a christening is used to indicate that a person is out of place in a particular situation.

These are, in chronological order, the earliest occurrences of the phrase as demure as a whore at a christening and variants that I have found:

1-: From A Fair Character of the Presbyterian Reformling’s Just and Sober Vindication of his Observations upon the 30^th of January, and 29^th of May, in Defence of the Reformer Rack’d. Being an Answer to J. G. G.’s New vile Rant, and the Weekly Observator’s invidious and false Reflections on it (London: [s.n.], 1695):

The Reformling would gladly be reputed Just and Sober in his Vindication, and if the World will take it upon the credit of his Title Page, or read no more of his Vertues, than what they may do Walking, upon a placarted great Gate, or a plastered Sign-post, he may be so still in a vulgar Eye. Sober! what? Because he has so out-run the Constable, But, stand in the Name of Justice; can he be Just too that does so? Well, however he sets a good Face on the matter, and carries himself Just and Sober in his Frontispiece, and as sure as Caudles are Caudles, he that will herd among the Sober Party, must look as modest and demure, as a Whore at a Christening.

2-: From The Recruiting Officer. A Comedy. As it is Acted at the Theatre Royal in Drury-Lane, by Her Majesty’s Servants (London: Printed for Bernard Lintott, [1706]), by the Irish playwright George Farquhar (c.1677-1707):

Plume. Soh——Now I must look as sober and demure as a Whore at a Christning.

3-: From The Spanish Jilt ¹, published in The Spanish Libertines: Or, The Lives of Justina, The Country Jilt; Celestina, The Bawd of Madrid; and Estevanillo Gonzales, The most Arch and Comical of Scoundrels. To which is added, a Play, call’d, An Evenings Adventures. All Four Written by Eminent Spanish Authors, and now first made English by Captain John Stevens (London: Printed, and Sold by Samuel Bunchley, 1707), by John Stevens (c.1662-1726):

This is the time of Danger, fly Girls from all Wit upon a full Stomach; as soon as the Clacks begin to run loud take to your Heels, and if you find them move Hands or Feet, Talk loud, which is as good as calling for help; yet if that will not do, look out and cry Nicky, Nicky, and I shall be sure to hear, and come in, looking as Demure as a Whore at a Christ’ning. Fear nothing, for when they see me come in so Grave, all will be hush’d.

¹ The Spanish Jilt is a translation of La Pícara Justina (1605), attributed to López de Ubeda (c.1560-c.1606).

4-: From A Vindication of the University of Cambridge. In Answer to a Scurrilous Pamphlet, Intituled, Animadversions upon the University’s Proceedings against the most Learned Richard Bentley, D. D. In which the Gross Falshoods, Inconsistences, and Blunders of the Author of that Pamphlet are Consider’d, and fully Exposed (London: Printed for J. Robarts, 1722), by a Lover of Truth:

He tells us in the very next Page, That he is loth to give Offence? But what has he been doing all this while? Does the calling the University’s Proceedings against such an Incendiary, (Pardon the Expression) Illegal, Arbitrary, and Unjust, contain nothing Offensive in it? The Terms, doubtless, must be Inoffensive, in the Mouth of a Person, who, after so much Billinsgate ², puts on the Countenance of a Whore at a Christning, wipes his Mouth, and pretends to be Innocent.

² Originally the name of one of the gates of London, Billingsgate came to designate the fish-market there established. The 17^th-century references to the foul language of this market are frequent, and hence scurrilous vituperation is itself called billingsgate.

5-: From A Seasonable Apology for Mr. H—g—r. Proving the Usefulness and Antiquity of Masquerading from Scripture, and profane History. With Observations on the several Species of Masks now in Use: And Likewise the Report from the Committee appointed to state and examine the Advantages arising from our present Masquerades (London: Printed for A. Moor, 1724):

That Masks are extreamly useful to almost all Sorts and Degrees of People, we know from daily Experience: It is to the Mask of Religion, that the Hypocrite owes his good Name; and to the Mask of Honesty, that the Knave draws well-meaning People into his Clutches in order to cozen them. It is through the Mask of Bravery, that the Coward is suffer’d to be saucy; and through the Mask of Demureness, that the Whore gets sometimes a new Spark at a Christening.

6-: From A New Dictionary, Spanish and English, and English and Spanish (London: Printed for J. Darby, A. Bettesworth, F. Fayram, J. Pemberton, C. Rivington, J. Hooke, F. Clay, J. Batley, and E. Symon, 1726), by John Stevens (c.1662-1726)—the literal translation of the Spanish proverb precedes the English proverb:

Tálamo, a bridal bed, or bed chamber. Greek, Thalamos.
Prov. Mesuráda cómo nóbia en tálamo: As demure as a bride in her wedding-room. As demure, as we say, as a whore at a christning.

7-: From Memoirs of the Life and Times, of the Famous Jonathan Wild, together with the History and Lives, of Modern Rogues, Several of ’em his Acquaintance, that have been Executed before and since his Death, for the High-Way, Pad, Shop-Lifting, House-Breaking, Picking of Pockets, and impudent Robbing in the Streets, and at Court (London: Printed for Sam. Briscoe, 1726), by Alexander Smith—the following is about a London prostitute who is accused of stealing a watch from a linen-draper:

The Linnen-Draper appear’d against her at St. Martin’s Vestry, where charging her upon Oath, before the Justices, of her Robbing him of his Watch: Quoth One of the old Mumpsimusses, Well, Mrs. Jelliver, what have you to say for your self now? you see the Fact is sworn possitively [sic] against you. Mrs. Jelliver, as he call’d her, dropping a very fine Courtsy, and looking as demure as a Whore at a Christening, said in her Defence [&c.].

8-: From Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings, Ancient and Modern, Foreign and British (London: Printed for B. Barker, A. Bettesworth and C. Hitch, 1732), by Thomas Fuller (1654-1734):

As demure as an old Whore at a Christning.