"Well," he said, pretending to think about it. "We both agreed not to die, right? So... as long as we live? Does that work? Or as long as you'll have me?"
He beamed at her, his smile bright and loving. "God. Isn't it crazy that you and I were born at similar times? I mean, for as long as humans have been around, you and I could have been born centuries apart, but we weren't. I can't believe I'm lucky enough to live at the same time as you."
"You could have been with anyone else," he said, looking down at her affectionately. "We could have never met."
"You wouldn't know I exist," he said, kissing the tip of her nose. "You'd have nothing to compare it to."
"Hmm, probably not. It'd be more like 'hey, that's a pretty handsome guy! What an imagination I have! I wonder if he exists' and then I would travel the world in search of you. I've always wanted to travel anyways."
"What if you never found me?" he asked, kissing her cheek. His eyes were bright with amusement, and he cocked his head curiously.
"Uh huh. So let me get this straight. If we had been born at different times, you would have imagined me, fallen in love with the idea of me, traveled the world to find me, even if I only existed in your imagination."