You and Papyrus became friends after you 'defeated' him in a battle, by flirting, what you didn't know about the innocent skeleton was that he was in love with you, a human and wasn't as innocent as he puts on, even he didint know it!

So one day, Sans had mentioned to him about intercourse... and he had no idea what it was... so he suddenly bursted into your house un snowdin and spoke.

"HUMAN!!! WHAT IS SEX AND INTERCOURSE?!" He asked in general confusion.