It's also the reason why it's so freaking adorable and beloeved by tons of people. It's chaste, pure love, currently unfettered by mere sexual passion.
If they had sex, or even kiss; it'd herald the end of the story, as most kisses and lovemaking scenes are wont to do.
Only series I've seen where sex and kissing the main girl didnt herald a incoming end of a series was thatBLue"Something" (gender?) series with the bugs and the mecha, and Outlaw Star.