They are fascinating to watch - not because he is unaware of sex, but because the opposite is true. David has been made aware of many sexualities and related acts, and there is nothing unusual in how Holloway makes love to Elizabeth.
The fascination stems from them specifically - how her skin yields to touch, where it reddens from the scratch of Holloway’s beard. It seems beautiful to David in a less distant way than the beauty of flowers or music; it’s something tactile, and he wants to mimic it - to see what marks his hands would leave on her skin.
Holloway covers Elizabeth’s mouth with his when she comes, stifling her moans, and David touches his own lips in sympathy before turning off the video feed and wondering what human arousal must feel like.