the one with the arranged marriage

Harry Potter has little to no hope of ending Lord Voldemort. And according to the prophecy, she's the only one who can.

When an opportunity arises in which she can settle the matter that doesn't involve killing him or being killed—Harry seizes it.

A marriage bond is the only loophole they can think of that could get both of them out of the prophecy. It's binding magic is ancient and powerful. Powerful enough to override that little line that said 'either must die at the hand of the other'. Now, neither can harm the other.

Besides, it's not like she imagined there would ever be a happily ever after for her where she marries someone she loves and has a family. The costs of this war are too great and her guilty conscience won't let her sit around any longer.