"I would remind the jury that there is no onus on the defendant to provide a reason why the boys would lie," the judge said.
"The onus is on the Crown to prove the defendant's guilt beyond a reasonable doubt."
He also told the jury they needed to consider each of the nine charges separately, although some evidence regarding a "pattern of behaviour" may overlap.
McGrath pleaded not guilty to the following charges:
One charge of sexual violation by unlawful sexual connection;
One charge of abduction;
One charge of performing an indecent act with the intent to insult or offend;
Five charges of sexual connection with a person under 16 years old;
One charge of sexual connection with a person under 12 years old.
The charges related to allegations of oral sex, sexual violation and masturbation.
The incidents were alleged to have taken place in a changing room which McGrath denied he had access to.
The defence alleged McGrath never had a key to the "premier" changing room at the rugby club where he was a coach.
The name of the club is suppressed.
The judge said in his summing up today there was no obligation on the Crown to prove "beyond a reasonable doubt" that the defendant had access to the area.
"They do need to prove that he had access, but not to that standard because it doesn't form part of the charges," he said.
Conflicting evidence had been presented earlier in the week on whether the defendant had a key to the changing room.
The young men were aged between 11 and 14 when the incidents allegedly took place in 2013 and 2015.
The second boy came forward when he was approached by detectives investigating alleged abuse of the other boy.
The judge reminded jurors of the Crown and the defence counsels' summaries yesterday.
Nick Webby, acting for the Crown, had said it would have been a "ridiculous coincidence" if both boys lied.
The defence had reminded the jury of the need for the Crown to prove the charges despite the emotional nature of cases of this type.
McGrath took the stand earlier in the week and said he had only ever conducted weigh-ins with the boys and taught them to "wash themselves properly".