Using the first line as the title is a very popular technique for a sonneteer.  But the title of the sonnet ‘If Thou Must Love Me’ goes far beyond that. Besides representing the first line, it reflects the theme of the poem very well. The poetess here emphasizes her views of genuine and sincere love and imposes her conditions on him if he has to love her. Failing to meet the conditions nullify his eligibility to love her. In other words, if he must love her, he should go the way she suggests. This is the central idea of the poem. And, the very title of the sonnet indicates the subject-matter well enough, making itself just and apt.