In chapter thirteen of Shane, by Jack Schaefer, Shane prepares to go alone to fight Fletcher on behalf of all the homesteaders with dreams, especially the Starretts. Shane and Joe Starrett have become very close in a short time, but Shane also has a strong connection with Marian and Bob Starrett.
Tonight Shane is "desperate with an inner torment" because he does not want to do what must be done: he must keep Joe Starrett from coming into town with him tonight. This kind of battle is not one which the strong, fearless Starrett can win, as it is going to be a gunfight. Mere talk will not be enough to deter the proud man, so Shane has to knock him out with the butt of his gun, gently, in order to keep the strong-willed farmer home.
Marian understands there is no shame for her husband in not being able to fight this kind of battle, but she has a question she must know the answer to before Shane leaves. She asks Shane if he is going to fight this battle just for her. It is a reasonable question, as there has been a connection, even a kind of love, between them since the beginning; however, Shane has too much respect for all the Starretts to have acted on his inclinations, and Marian loves her husband too much to consider being unfaithful.
Shane looks at Marian before he answers with the question you cite: "Could I separate you in my mind and afterwards be a man?" Shane is telling her that he could never do anything which would harm this family, his friends, and what he does for one he does for all of them. What she asks him is whether he is going to fight this battle because he loves her; what Shane tells her is that he is fighting this battle because he loves all of them.