McDermott, 47, and Spelling, 40, have been married for seven years and have four young children together. They also have starred together in numerous lifestyle TV docu-series.
Spelling suffered life-threatening complications from the birth of their youngest child, 16-month-old son, Finn.
Emily Goodhand told the magazine she had sex with McDermott and spent two nights with him in Toronto while he was in town early this month promoting his cooking show, "Chopped Canada."
"He told me he and Tori had a sexless marriage," said Goodhand. "I believed him."
UsMagazine.com said publicists for McDermott and Spelling declined to comment on its story.