Sperm Donation, why is it right,ethical when the child born does not even get to know his/her biolgical father?

Being a parent i have come to really understand the need and importance of both mother and father. It is unfortunate enough the world is full of kids with both or either due verious circumstances. What i dont understand is the men to go out and donate sperm. I believe it is wrong as knowingly the true identity of that child is taken away. He or she is raised knowingly raised by the strangers.