I actually thought it was very obvious why.
She is totally in love with Tom Hank's character, and she knows her heart is going to be broken because he is never going to stay there forever, so from the very first moment she starts to fall for him she instantly starts purposely trying to distance herself, she tried to shun all his initial attempts to speak to her, and even after they got together she asked him to never ever come back again when he left for war. She asked him to never come back and to never write.
Its all because she is afraid of being hurt in the end. The fact that he is not the correct religion is just an additional problem.
So then they have their final night together, and he has to leave for war. She instantly runs to the guy who loved her all along, she wants to forget all about it and immerse herself in a stable relationship where she will make herself forget Hank's and make herself live in a 'safe' 'secure' marriage.
Of course, she eventually realises she is only fooling herself by doing this and marrying him...
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