Celie's reason for wanting Nettie to marry an older man can be summed up in the following statement:
Just the thought of anybody getting pregnant make me want to cry.
Celie loves Nettie dearly and understands her high intelligence and great potential. She knows, too, from her own life that pregnancy is debilitating. She has also watched the women around her, such as her mother, become worn out and used up by pregnancy after pregnancy. Too many children, too soon, consumes a woman's soul with having to care for them. This is a theme that Celie returns to over and over again as she thinks through what her, or any woman's, life means.
Celie would prefer—and works to enable—Nettie not to marry too soon. An older man like Samuel is her preference for her sister because a man like Samuel is wise and mature. As an older man, he is not driven by the lusts and need for self-assertion that Celie has repeatedly experienced in her interactions with younger males. Samuel is a good companion for Nettie after his wife Corinne dies because they live, as he explains to the bishop, as "brother and sister."
Celie has come to understand that in order to experience joy, women need to take care of themselves and protect their own potential by avoiding endless childbirth.