The youngest alleged victim of former BBC Radio Leicester presenter Julie Wadsworth told a court he was aged about 11 when she asked him: "Do you want an experience?"
The man, now in his 30s, told Warwick Crown Court he was sexually touched by the radio presenter.
He described to the court how Mrs Wadsworth was wearing a strappy top and short skirt but no underwear when the alleged offence took place.
He said the couple must have known he and his friends were underage.
The man said he had already watched Wadsworth and her husband Tony "have full sex against a tree" after spotting her sunbathing topless in a park.
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He claimed Mrs Wadworth later had sex with one of his friends after she told him to come back another day "with protection".
"There was a woman sunbathing topless with a man and I didn't know who they were," the man said.
"He was just sat down with her and he was fully clothed.
"I was with my brother and his mates and we asked what they were doing.
"I remember me saying 'get your boobs out' and she did a couple of times.
"I don't think the fella felt comfortable with me being there and me being so young."
The man said he originally told police he had not had a sexual experience with Mrs Wadsworth because he was too ashamed.
"I had my kids in my house so I was embarrassed and nervous," he said.
As it happened: Evidence starts in indecent assault trial of former BBC presenters
"But me and my friend did have sexual experiences.
"The first time I saw her was when I was with my brother and some of his mates. I was probably about 11.
"My brother was chatting with her but that is when they actually had sex in front of us.
"They had full sex up against a tree. We were about 10 metres away.
"They knew I was there but didn't ask how old I was.
"There were four or five of us."
Julie Wadsworth (60) and her 69-year-old husband, from Broughton Astley, deny indecent assault and outraging public decency.
The trial continues.