"Pretending sodomy" here is just that: claiming that 'sodomitic practices' have taken place, with the accuser starting or 'pretending to start'/feigning to begin such an act.
This means that someone accuses another one of having engaged in sodomy, as a way to ruin their reputation, even after the death of the accused. Or in this case as a second option, on top of threatening to perhaps fatal violence against the victim.
This was one form of blackmailing.
Whether anything real in terms of homosexual activity occurred was immaterial to the accusation, whether the so accused had any inclination to engage in it had no relevance. The threat of being painted in that way before the public eye was enough.
This specific incidence is detailed in the court proceedings:
Trial of Stephen Margrove and John Wood
Stephen Margrove and John Wood, of the Parish of St. Martin's in the Fields, were indicted for Assaulting George Smith on the King's High-Way, and taking from him 22 s. the 18th of January last.
The Prosecutor deposed that the Prisoners came up to him (and John Wood took him hold by the Collar of his Coat) and demanded his Money, and said if he would not give it them they would take away his Life and swear Sodomy against him; that by means of this Violence, and being under a Terror, and in great-Fear he gave them what he had in his Pocket, which was Half a Guinea, and about 11 s. or 12 s. in Silver but they not being contented with that said they would shame him if he did not, give them more Money, and went Home with him in his Master's House in Golden-Square , where being in a Surprize, he gave them another Guinea; he was sure they were the same Persons, he having seen them before, when he was with his Master at Tunbridge.
The Prisoners pleaded that the Prosecutor came up to Wood while he was making Water, and took hold of his Yard, using some unseemly Expressions, whereupon he called out a Sodomite; that then the Prosecutor fell on his Knees, and begg'd them not to expose him, and took them over to the Hoop Tavern Door, and gave them half a Guinea and some Silver here (which the Prosecutor deny'd) they said further than the Prosecutor took them Home to his Master's House and gave them another Guinea; and urg'd that it could not be robbing on the High Way, because the Prosecutor gave them the Money; But the Court observed that the Money, they took from him in the Street, was mention'd in the Indictment; and that he that took a Man by the Collar in the Street, and demanded his Money with threats to take away his Life if he did not give it them, and by such violence and putting in feat got his Money from him was guilty of Robbing him.
They called several to their Reputation, by which it appear'd that Margrove had been a Servant at Young-Man's-Coffee-House 6 Years, and Wood had been a Gentleman's Servant, but none to give an Account how they spent the last 6 Months of their Lives.
The Jury found them Guilty.
For the defendant Wood:
That of late he had made it his Wicked Practice, by using some undecent Gestures, to induce Men to Sodomy, whom he fancy'd might be inclinable that way, as if he would have yielded his Body to that foul Act; but when he had taken the Money agreed upon beetween them for it, he deceived them, and told them, that he was not for their Sport; and he must have more of their Money, or else he would accuse them, and defame them When he now came to himself, and consider'd this, and other his heinous Offences, he express'd great grief for them, as Stephen Margrove, (hereafter mention'd) did for his, who had committed the like Undecency and Cheat.
The accused Margrove told a similar story:
But after his loud Protestation of Innocence as to this Robbery, he at last confess'd his being concern'd in it; adding, That he had often allured Men (whom he supposed to be given) to Sodomy, but never suffer'd them to proceed to that foul Act with him, his Intent being only to get some Money, which he extorted from them by threatning them, much in the same manner as John Wood did. He acknowledg'd this was a great Offence, and therefore humbly begg'd Pardon of God, and all he had offended, to whom he could make no other Satisfaction.
Source: The Proceedings on the King's Commission of the Peace, and Oyer and Terminer, and Goal-Delivery of Newgate, held for the City of London, and County of Middlesex, at Justice-Hall in the Old Bayly, on Thursday, Friday, and Saturday, being the 14th, 15th, and 16th, of this Instant May, 1719. In the Fifth Year of His Majesty's Reign; and the Account of the Ordinary of Newgate, London: Printed for Samuel Briscoe, at the Bell-Savage on Ludsgate-Hill, 1719.
— Rictor Norton (Ed.): "Trial and Execution of Three Blackmailers, 1719", Homosexuality in Eighteenth-Century England: A Sourcebook, 2 Jan. 2011, updated 16 Aug. 2021 http://rictornorton.co.uk/eighteen/1719blac.htm.
An alternative retelling of this incident is found in the early biographical sketch book — Captain Alexander Smith: "A Complete History of the Lives and Robberies of the Most Notorious Highwaymen, Footpads, Shoplifts and Cheats of Both Sexes: Previously published 1719 and 1926", Routledge: London, New York, 2013. (older versions on gBooks).
Perhaps noteworthy: the accusation against the defendants Wood and Margrove was first assault then robbery then 'pretending sodomy', in this case to extort money. As noted in The Historical Register of 1719, the reason for execution on May 16 was simply given as "for robberies on the Highways" (gBooks)
This latter 'threat' was becoming seen as a common 'type' of crime:
See for example this similar collection of events in
— "A Genuine Narrative of all the Street Robberies committed since October last, by James Dalton, and his Accomplices … To which is added, a key to the canting language, occasionally made use of in this narrative. Taken from the mouth of J. Dalton", by James Dalton (street-robber), J. Roberts, 1728. (gBooks)
or these entries on 'threats' and 'sodomy':
Accusing, or threatening to accuse, another of a crime, with view of extorting money,& c. […]
THREATS. (See tits.“Accusing," "Letter(Threatening)," and “Sodomy.”]
318. [The abominable crime of buggery committed either with mankind or any animal.] (See tit.“Assaults,” ante,p.550,Offence 37.)
319.Accusing - or threatening to accuse--thereof, with a view to extort money, and thereby extorting same.
(See tit. “ Accusing," ante, p. 544, where money extorted;and “Letter (Threatening),” ante, p.588.)
— George Colwell Oke: "The Synopsis of Summary Convictions, showing, at one view, the penalties, &c. for 1300 offences, when proceedings must be commenced, what justices to convict, &c., with introduction, practical observations; an epitome of other matters coming before Justices out of Sessions and an abstract of the Juvenile Offender's Act", Butterworths: London, 1857. (gBooks)
So a quite similar approach to the case is found in more generalised form in:
The general sensitivity on the subject of homosexuality led to extortion of money under threat of exposure for unnatural vices becoming a recognized form of crime, though it was not made the specific subject of a criminal statute till 1826 (6 Geo IV, c. 19). As early as 1728 it was reported that men pretending to be male prostitutes solicited in the London streets with the intention of blackmailing anyone who responded to their advances, but in later years it was alleged that most people who brought prosecutions for this form of extortion were in fact practising homosexuals trying to cover up for themselves.
— A. D. Harvey: "Prosecutions for Sodomy in England at the Beginning of the Nineteenth Century", The Historical Journal, Vol. 21, No 04, 1978, pp939–948.