To me, the theme of Sonnet 138 is the idea that love, or at least lust, is pretty tolerant. Or perhaps it fools itself -- maybe these are the same things... This is especially true as one grows older and has fewer opportunities to satisfy one's lust.
In the poem, the speaker "believes" his love, even though he knows she lies. He tolerates the fact that she is unfaithful. He knows that he is getting old, but he pretends that she thinks he is young and sexy.
Through the whole poem, it is clear that both lovers are sort of fooling themselves because they just want to keep sleeping together.