Marshall wasn't some dashing prince, prideful in need of saving. Fionna enjoyed that. It made him enigmatic. Ethereal. Immortal and unattainable, and yet, hers. He was a testament to times far gone, and yet he was the centerpiece to her life in their time. He'd been alive for as long as the world hadn't. She saw it as proof of his inevitability. He would always be, and as far as anyone could be concerned, always had been. And he would continue to be in her own life, until the life was sucked from her bones by cold death, and they were no longer each other's.
She'd asked him once, to do it. He denied her. He wouldn't be the one responsible for taking away the world's Last Human.
So, for as long as she lived, she endured to fight with him, in day or night, and lay with him when he lay. And she would fall long before he fell.