If by "remember" you mean an exact and accurate recall of the associated physical and emotional sensations, then here's my take.
Pain: If I could recall and relive the pain, I shall do everything I can to prevent a recurrence. It's because I cannot recall and relive that I fall into the trap of repeated painful experiences.
Sex: If I could recall and relive an intercourse in full, I might not need to go through the process of experiencing the next one. It's because I cannot recall and relive that I desire the experience again.
Think about it.