That is completely not true. Especially if it was handled in the same tactful way it's always been addressed in the books - through subtle words and actions that don't detract from the main narrative. Also it could add to the idea of the Spartans coming to terms with the fact that they can live normal lives outside of battle (as we have seen others like Maria and Randall do), and can heal. Now is it ultimately something "has to happen" in the story - no. But if it did it would certainly make sense and build on certain worthwhile character developments and themes.
And it would create a story even worse than Halo 5's. We don't need a romance between John and Kelly, it would add nothing substantial to the franchise.
A possibility that won't come to pass.It might
That's the thing though, in the books it's always been a subtle hint, no more than that. They never outright said "John and Kelly are in love and want to get it on", it was signs of a close bond. That bond could be a friendly bond, familial bond, romantic bond, it's never specified, and it shouldn't. That level of mystery keeps their bond interesting, and developing it into something romantic would be ruining it.
And yes, Maria and Randall got to live a normal life, but they are not Spartans anymore. Randall had some of his augmentations removed, and Maria basically retired from the service. As a Spartan-II, you're basically just fighting non-stop. Sure, there is R&R, but I seriously doubt that the first thing on their mind is "Omg I would so tap that other Spartan".
Now, if we're talking about Spartan-IVs, then it's a completely different story. We see the whole romance thing between Buck and Dare. And you know what? That's interesting, especially since it serves as a contrast between Spartan-IIs and Spartan-IVs. While the IIs are undoubtedly human and have strong personalities, their lives are dedicated to being soldiers, their priorities are fighting the good fight. But Spartan-IVs? They live normal lives outside of their job.
Plus, even if it were to happen, I seriously doubt 343i's ability to properly handle a romantic relationship between John and Kelly. As shown with Halo 5, their ability to characterize Blue Team is extremely poor and does not do them justice. Why would this be any different?