oil leaking from the output shaft seal, gear shifter seal, and the drain plug
The oil "loss", as long as it's slight, and you keep an eye on the level will not damage the engine. As tx said, oil getting onto the rear tire can make for some unexpected, exciting moments.
I would also be worried that instead of leaking seals, maybe you have a crack in the case near the sprocket. Clean it well and look. If no crack, you should be able to check and fix the drain plug without too much trouble.
For the seal leaks (once you confirm the seals are leaking, not the case) you can try going up a grade in engine oil viscosity, ie: to a 20W50, mineral based (not synthetic) and shorten the drain interval. The extra VII, and keeping the oil fresh, make help reduce the amount leaking from the seals.