Unless you're just not compatible sexual role wise (I believe that CAN'T be changed), then good sex can be taught.
I didn't believe that until I was seeing this guy earlier this year. Sweet and handsome all the way around, the first time we had sex... HORRIBLE! The thing that was cool is that he asked me how it was and I let him down easy by saying, it was ok, but could be a lot better. I gave him my thoughts and I guess he took mental notes because the next time was ABSOLUTE FIRE! Whew! Ddy knocked it out the fucking PARK! We would still be together if I didn't move. I think about that sick and stroke quite often...

So yes. It is important. unless they are just stubborn, or incompatible to you (in that case you don't need to be with them anyway), teach and get your life Prof. Cack!