I headcanon that on Tatooine small families are the norm. With food and water and well, anything that isn’t sand being scarce having to provide for more than three or four people at a time would be a nightmare. So the women of the desert have passed down a wealth of contraceptive recipes and techniques from generation to generation.
When Beru and Owen Lars accepted the son of a man they’d met once, they knew that this was it. The only child they would ever raise. They could tell themselves sweet lies about how maybe the harvests would be plentiful for the next few years and they could get by, but they are children of the desert. They cannot refuse reality in a place as harsh as the desert. It would only serve to drive them mad or make them suffer. They could have said no. They didn’t really know his father and they know the danger will be great. They could have said no. They want their own baby and cannot have two. They could have said no. But how could they refuse the grandson of the woman they loved so much? They could have said no. But there he is, small and beautiful and helpless and in need of a loving home. So they accept him knowing that he is the only son they shall ever have. They love him as the only son they will ever have. And he loves them dearly.