Well, if you didn't notice, remember, Chise is not human, they could never had done it unless it was right off the bat, first episode. So...they couldn't all along

Dude if that was the case they wouldn't have been able to do it in the end. Chise was so shy about her body that maybe they would get close to doing it before but it was only it the end that she was willing to go all the way.

imho

the scenes where they were in bed together they were making out and just 'fooling'around in their words which means in basic terms 'foreplay'but no sex.

so they never actually had sex until the last episode.