The gemarah (nedarim 20b) mentions that one of the reasons for having beautiful children is due to having relations "as if forced by a demon". Rashi there explains the term as either indicating that the act is performed forcefully or in a completely covered way as if hiding from a demon. This is codified in the halacha (shulchan aruch, orach chaim 240:9) where it is explained differently as either performing the act in fear or under duress.
I would like to understand this concept
- How did this go from being a 'segulah' (charm) for having beautiful children to a compulsory halacha?
- What is this business with demons? If the intent is to perform the act in any of the ways described above, why not just say so? What relevance do demons have over here?
- Why does the shulchan aruch explain the implications of 'being forced by a demon' differently than Rashi? Where does he get this explanation from?
- Why, in fact, must one perform this mitzvah in a way that wold seem like it is against his will? Usually we are encouraged to be happy in our performance of mitzvot, why not here?
- What is the woman's role here? Should she also feel compelled by a demon?
- This does not seem to conform with the behavior of Rav when he slept with his wife (brachos 62a).