They resolve to do whatever they can to not think about each other that way. After all, their parents' relationship is what matters the most. Who are they to get in the way of it? They can surely control themselves... can't they? But they quickly realize that as much as they can try to ignore each other... the attraction is just too strong. They have to figure out a way to quell their desires- without clueing their parents into it, of course.
One day, while their parents are gone, they come up with a plan. They never had sex with each other while they were dating, so maybe they NEED to have sex with each other NOW to get it out of their system once and for all. It might be the only way for them to live normal lives together. A taste of the forbidden fruit before they chop down the tree altogether. But will this one instance of passion quell their desires, or simply fan the flame?