The discrimination Lupin faced and dealing with the horrible idea of possibly hurting someone he cared with while transformed certainly bothered Lupin and why he was so afraid about getting married and then having a baby that he might pass it on to to begin with (regardless if that were superstition or real). Then, when he saw the baby he loved him and demonstrated the normal new father thrill and the fear was gone. It wouldn't have mattered because Teddy was loved.