These lines come infinitely close each other, so at some level you will always have that effect as they have finite thickness. Maybe this helps a bit:
k = Pi/16;
contours = Table[psi, {psi, -Pi, Pi, k}];
sl[x_, y_] := -(Exp[x] Sin[y])/(Exp[2 x] - 2 Exp[x] Cos[y] + 1);
ContourPlot[sl[x, y], {x, -Pi, Pi}, {y, -Pi, Pi},
ContourShading -> None, Contours -> contours, PlotRange -> All,
ContourStyle -> Thickness[.001], PerformanceGoal -> "Quality",
PlotPoints -> 25, MaxRecursion -> 2, ImageSize -> Large]