The Springboks dominated the second-half, crossing the line through captain Mandisa Williams, Lorinda Brown and Zenay Jordaan, with only Lucy Millard touching down for the visitors.
And Parker was a frustrated figure after the final whistle, bemoaning a lack of playing time for his side's poor display.
He said: "Today was disappointing. We looked like a team that hadn't played together in two and a half months - we hadn't played together for that time.
"South Africa looked more cohesive. We defended well in spells but made too many mistakes. We were beaten by a better side."
Scotland went ahead after a minute as Melrose full-back Nicola Halfpenny slotted over a penalty, but the scores were level just eight minutes later as Yolanda Meiring converted one of her own.
And the Springbok's pressure told before the interval as Siyolo crossed the whitewash, but the conversion was missed and they took a five-point lead into the break.
South Africa upped the pace straight from the restart and Williams found a gap in Scotland's defence to score, only for her team mate Lamla Momoti to get yellow carded minutes later.
This let Scotland back into the game as they took advantage of the extra player and outside centre Lucy Millard brought the scores to within five points again.
But the visitors' power told in the latter stages and Brown and Jordaan both touched down for the convincing win.