Pre-game they're already engaged through political convenience but they've hardly spent time with each other, other than for Noct's upbringing. Then the roadtrip happens, they end up spending way more time together than they expected, and they discover that their arranged marriage might not just be a bad idea.

Slow burn is good and if you can fit porn in, I'd be deeply grateful but porn is not necessary. I just really need this in my life.