Former Singapore Civil Defence Force (SCDF) chief Peter Lim Sin Pang is deemed to have obtained gratification from the representative of an SCDF vendor, as soon as she gave him oral sex.

This was regardless of whether the act was completed, said the prosecution in its closing argument on the final day of the sex-for-contracts corruption trial on Tuesday.

District Judge Hamidah Ibrahim reserved judgement, and the verdict will be heard on May 31.

Lim, 52, is charged with corruptly obtaining oral sex from former Nimrod Engineering general manager Pang Chor Mui in exchange for furthering the business interests of her company.

Defence counsel Hamidul Haq, however, disagreed that the oral sex was an inducement, and told the court that the prosecution had failed to show that the sex act was anything but "innocuous" and the "culmination" of a close friendship between Lim and Ms Pang.

"The fact remains that when the pants were unzipped, she had the option of doing nothing... She chose to do oral sex." said Mr Hamidul, who added that Ms Pang had performed the act "of her own accord" and with "absolutely" no understanding that there would be any exchange of favours.

In a rebuttal against the defence's submission that the sex act in May 2010 was not "completed", Deputy Public Prosecutor Tan Kiat Pheng asked rhetorically: "If a buyer were to buy lunch for the receiver, and the receiver was not satisfied with the lunch, is it then that gratification didn't take place?"

DPP Tan added that the defence was trying to "trivialise the oral sex" by saying that it was between close friends

"(Lim) knew he was obtaining oral sex from a representative of a vendor, and from no other than the GM, the de facto second person in the company," said the DPP.