By BILL MILLER A gymnastics coach from Lewisville has been jailed on criminal charges resulting from an alleged sexual relationship he had with a male juvenile athlete.Marcelo Silva Guimaraes, 38, was being held Friday in the Denton County Jail on four charges of sexual assault of a child, with bonds totaling $300,000, according to jail records.Guimaraes was arrested Monday by Flower Mound police, said Capt.