Why do they do this?

I found the very limited nudity neither off putting nor unwarranted in the scene. It does signify Sarah becoming a woman. It is not done in a particularly voyeuristic way. The camera does not focus on them, but rather the breasts become an element showing a transition in her life, along with what appears to be blood stains in the bed. It speaks directly to Sarah's changing from a chaste young woman to a willing lover, which is an important step in the evolution of the relationship. Remember that she will leave while he is still asleep. We, the audience, need to know what happened for the next sequence of timidity, uncertainty and realization to make sense.

Good coffee and bad movies are my hobbies.

The scene was completely natural. Two people who just were intimate are relaxed around each other. What looks contrived are (what you seemingly want) the magic sheets that adhere above a woman's tits, but rest at the man's waist. But showing two people who are falling in love and have just been together in bed, comfortable, not caring if a tit is exposed is somehow "completely pointless!" as you find it. Have you ever actually been in bed with a woman?