A 12-year-old alleged last week McGrath told him to masturbate to relieve stress before a game before showing him how to do it for "about two seconds".
McGrath told the court today: "I didn't ask him to do anything with his penis."
He said his grandson and the boy had been trying to lose weight before a weigh-in and after running in the rain they went to the club rooms to step on the scales.
McGrath said he wanted to check their accuracy so jumped on the scales fully clothed but thought: "It can't be right".
So he took off his boots, socks, denim work shorts and oilskin jacket, leaving his T-shirt on before trying again and thought they were then "pretty good".
He said he told the boys to strip down and hop on the scales.
"They were in the nude. That was a nude weigh-in."
McGrath said he believed that was common practice in rugby and league.
After taking a mental note of their weights, McGrath said he told the boys to go shower and "wash properly".
He said the complainant looked confused so he gestured with his hands towards his penis and said "for the hygiene".
"I'd always told the boys to make sure they do their hygiene."
Last week, the jury watched a specialist police interview with the boy from November 2015 when police were investigating the other allegations.
The boy said McGrath told him to masturbate after the weigh-in.
"[He told me] you can do it in the shower whenever. He told me it was to relieve stress and stuff like that."
The coach then allegedly masturbated for "about two seconds" and told him to do the same, which the boy said he did so he "could get out of there".
Under cross-examination by Crown prosecutor Nick Webby asked McGrath if he could think of any reason why he'd say these things about him.
"No I cannot."
McGrath said he never spoke to the boy about reliving stress by masturbating as he would never have used the word as it wasn't in his normal vocabulary for the act.
Defence counsel Siobhan Buckley asked McGrath about what the second complainant alleged in 2015.
The boy, then aged 14 years old, said the former coach showed him how to masturbate and sexually violated him in the club rooms to make him "more mature".
But McGrath said today under cross-examination those things never happened.
"I did not do that. I was never in that room with that boy. I had no key."
He also said he did not sexually assault the boy after he was injured on the field.
McGrath said he told the boy to go stretch in the locker rooms.
"I went back to the room and I just grabbed the bottle of body wash or shampoo or something and I used that and I massaged it," he said before gesturing to his own hip in the stand.
McGrath said he used shampoo to help his fingers glide over the skin and that he'd rubbed the boy's hip to "try get the injury up off the bone", a technique he'd learned many years ago.
Webby put all of the allegations to McGrath who denied them all because he wouldn't have had time and need to get back on the field.
"I don't know why he's made these allegations and lied about it," the defendant said.
The rugby club McGrath was involved with at the time and the scene of the alleged crimes are suppressed.
The trial continues.
