As we've known for weeks, the whole argument began when Luke misguidedly believed that Hannah felt the same way about sex that he did. He's been practicing abstinence until marriage after a literal come-to-Jesus moment in college. Hannah, however, had just had sex with Peter in a windmill as is her right as a consenting adult looking for her husband. The fact that Luke just assumed Hannah felt the same way as him because they're both religious was super out of line. Hannah told him as much, saying she didn't agree with him about his views on sex before marriage. But Luke wouldn't just shut his mouth. He then tried to backpedal saying he would understand if Hannah had a "slip up" and that they'd work through it, but also not if she "slipped up" with all the men. The conversation went quickly downhill the more Luke spoke until Hannah had enough.