In Letter 46, Shug angrily denounces Albert's behavior towards Celie, stating that she would be a better spouse to her than Albert. This is an important instance of dramatic irony. Shug does not know that Celie has sexual/romantic feelings for her, a fact that readers have already been made privy to:
Anyhow, once you told me he beat you, and won’t work, I felt different about him. If you was my wife, she say, I’d cover you up with kisses stead of licks, and work hard for you too.
Shug's hypothetical is not so hypothetical, after all—especially given the knowledge that the two women do, eventually, enter into a sexual/romantic relationship.
Thematically, Shug's statement is intimately connected to Walker's contemplation of gender roles in the novel. Shug discusses a potential marital relationship with Celie in purely theoretical terms, framing this as a condemnation of Albert more than a confession of her own feelings. In a heteronormative society with strict gender roles, queer people are often forced to state their feelings indirectly to avoid violence and ostracism. Shug's theoretical marriage to Celie might simply be a figure of speech. It is equally likely, however, that Shug makes this statement to test the waters with Celie, feeling out the other woman's amenability to a queer relationship.