Caleb Horner, 38, was sentenced in Melbourne's County Court to a three-year community corrections order for two counts of sexual assaulting a child under 16.
He was handed a 274-day jail term but has already served that in pre-sentence detention.
Judge Stewart Bayles reiterated multiple times throughout his sentencing on Thursday that Horner's offending was serious, but noted his previous experience in prison and his willingness to engage with rehabilitation.
"The sentence imposed should not be regarded as soft," he said.
"It is intended to be an onerous order with significant conditions and consequences."
Horner was accused of grooming a 14-year-old boy after striking up a conversation on Instagram in 2020.
The two met on February 17, 2020 with Horner collecting the victim in his car and driving to McDonald's to get food before parking at Cheltenham Park in Melbourne's southeast.
There, the prosecution said Horner, who was 34 at the time, began feeling the victim's leg and put his hand down the boy's pants.
When the boy pushed his hand away, Horner told him it "would be fine" before forcing the teen to put his hand down his pants despite his resistance, Judge Bayles said.
Horner was arrested in November 2021 at his Perth army base before being extradited to Victoria.
He admitted exchanging messages despite knowing the victim was 14 , but denied anything sexual occurred other than kissing, the judge said.
He was released on bail before a jury trial in 2023 where he faced 20 charges but was acquitted of 18.
The judge noted Horner's relationship with his family deteriorated after he came out as gay, which also led to him being excommunicated from his Mormon church.
His time in prison was more burdensome, having been seriously assaulted in his cell and requiring prison protection.
It was also during an intense part of the COVID-19 pandemic, during which Horner spent three of the nine months in isolation.
He was formally discharged from the defence force in April 2023, but the judge rejected this was extra-curial punishment.
The 38-year-old has been ordered to complete a three-year corrections order, with 250 hours of community work, mental health treatment and programs to reduce his risk of reoffending.
The judge declared his prison sentence already served and he will report as a sexual offender for eight years.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
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