Michael Lowenthal’s new story collection, “Sex With Strangers,” is nimble in its particulars. The stories take place in gay clubs, on cruises, along beautiful beaches and in humble small-town kitchens. His characters are men and women, gay and straight, at home and abroad, beautiful and less beautiful than they once were. There’s an ease to his storytelling, too. Nothing feels strained, and the stories slow down and speed up until their climaxes arrive with a weirdly deadening ambivalence.