One Direction preference

if you're single when you're both a certain age, you'll marry each other.

Harry: "Harry's just my friend." You insisted, laughing at yet another of your friends who claimed you were both in love with each other. "I swear, the only way I'll end up with Harry is if I'm fifty and still single. We're best friends." You looked down at your homework, hoping to resume studying. "So that's the deal huh?" You friend asked, smiling to herself as you nodded. Though there was a deal in place and you believed Harry to be just your best guy friend, it wouldn't be long before you discovered something more, both of you to be wed by the time you were half the age of your proposed deal, never needed as you fell in love all on your own.
Louis: It'd been come a sort of all-around joke. You'd always referred to Louis as "husband" as he called you "wife". Somewhere along the line you'd both agreed that one day you'd be married. Well, as long as you didn't manage to find someone else. You both travelled through life with boyfriends and girlfriends here and there, one nearly turning into a marriage for you. Yet, it seemed that you'd still manage to find your way back to Lou. Almost as though he were a constant. It was an unspoken deal you two had, after fifteen years of knowing each other and still not having found the one you both wanted to spend the rest of your life with, you realized he'd been right in front of you all along, making him your actual husband after you found that there was no one else.
Niall: Niall had been your first relationship, your first love, your first everything. You sat with your bags packed as the pair of you would be truly separate for the first time. You both knew this would be a test but you also thought you could do with some time apart, not wanting something completely serious as you headed into your first year of college, especially since he wouldn't even be there to share the experience with you. "How about we wait? We'll wait until I finish with school and until your career slows down a little. After that, if we're still in love and want to be together we will." You suggested, Niall's hands wrapped around yours as he thought about it. "Marry me then." He stated, your brow furrowed as you'd just suggested you take some time apart and he was proposing. "I mean after that. If we come back and we still feel the exact same about each other. If we make it even with the time and the distance, marry me." He repeated the words. "Okay." You nodded, one final kiss goodbye as you went your separate ways. You couldn't help it, noting all throughout school that Niall had remained single, unable to help but wonder if he was waiting for you, if he still loved you. Only time would tell as six years later, you walked down the aisle, your first and last love waiting at the end to seal the deal you'd made so many years ago.
Liam: You leaned into Liam's shoulder, tears staining his t-shirt as you cried. "No one's ever gonna love me. I'll just end up being alone forever. Pathetic right?" You laughed through your tears. Liam sat beside you, tilting your chin up so that you were looking at him. "That's not true. I won't let it happen. You want to be married before you're twenty-eight right? Well then, if we make it there and we're still both lonely we'll get married." He said, some of the emotion behind his words true. Truth is, he'd been in love with you for years now, afraid of ruining our friendship with such a confession. "Alright." You answered, unaware that in just a few short months he would become a part of a boy band that would change his life and stunned when he still kept his promise so many years later.
Zayn: You sighed, a single blow leaving your lips, putting out the tiny candle on the cupcake you'd brought yourself for your birthday. Your small apartment was empty other than the small touches of yourself that you kept around, a loner's home as you so called it. A knock on your door interrupted your thoughts and you wondering if again someone was knocking on the wrong door. A bouquet of flowers covered the face that stood on the other side, a vaguely different yet familiar accent filling your ears as he spoke, reminding you of the words he'd said so long ago. "I promise that if we're both thirty and not married yet I will come get you, make you fall in love with me, and then I will marry you." He lowered his hands and there he was again, Zayn Malik, ready to uphold the promise he made to you once before, never forgotten and to be the marking of the final birthday you would ever spend alone.