Final answer:
John's desire to marry Lenina clashes with her expectation for casual relationships, leading to her exasperation.
Step-by-step explanation:
John wants to marry Lenina, and she gets exasperated because she is not accustomed to this kind of commitment. In the society Lenina comes from, the concept of marriage is obsolete and relationships are encouraged to be casual without long-term bonds. This cultural difference causes tension between John's romantic ideals, which include commitment and marriage, and Lenina's conditioning to engage in casual relationships.