A life for a life, that is essentially what Arashi has given to Sorata.
Sorata's destiny is that he will protect and aid Kamui in every possible way available to him, but Sorata will die for the woman he loves. Upon meeting Arashi, he decides right off the bat to choose her. He is quite literally giving his life for Arashi. But she in turn has given her life to him, if not in the literal sense.
From the time she was seven years old, Arashi has been raised to be a Dragon of Heaven, a protector of Kamui, and a holy shrine maiden. This was her entire way of life until she met up with our charming young man from Osaka. She tried to fight against her feelings, but eventually she could no longer hide what she felt.
She gave herself over completely to the feelings she had tried to keep submerged for so long, and in doing so, gave herself completely to Sorata.
Being no longer a virgin, no longer pure in the sexual sense, Arashi has lost the ability to call upon the powers of Ise. She can no longer unleash the sword, nor can she create a kekkai. She can no longer continue with the life that she previously had, she has to start completely over, and maybe now, with a new mission.
If you think about it, Arashi's sacrifice was maybe even more harder to give than Sorata's may be. For even though dying in the name of Love is one of the more noble sacrifices, continuing to live after you have lost everything may be the hardest yet.