Why do Krissy and Billy get married in the novel, All Good People Here?

When she gets pregnant on accident, she knows that Luke is the father, but she goes to Billy to ask for abortion money because she knows Luke does not have money. Billy misunderstands her and proposes to her. In a spontaneous decision, Krissy decides to agree to the proposal so that she can stop being so poor and live a comfortable life as part of the wealthiest family in town.