That's the point -- whether their relationship "stays more boring" is entirely in your hands and down to your choices. So it's super-weird to complain that's how it's going to be if that's what you choose it to be.Hahaha, I win what? The only thing I won is the end of that cringy flirting and I know a more boring relationship is waiting ahead. See, if they can stay together? I thought it was all in my hands. I decide if they cheat or not and even if they do, I'm sure they'll forgive each other.
"I'm sure they'll forgive each other". They will, if you choose to. But if you consider such outcome boring, why would you choose it? To bore yourself?
Never mind the silliness of claiming you already know what choices you'll make in a story when you don't yet know what's waiting in the store.