OK, there's a lot in this Letter so we'll go one step at a time...
Of course, as with others of Lewis' books, we only see one side of the conversation. Here we do not see Malcom's letters, so we don't have an exact formulation of the parallel he attempted to draw between prayer and a man making love with his wife. Yet, it's clear that Lewis, rightly so in my mind, is not at all impressed. Two points to make on this part.
1) Jewish literature and liturgy uses many parallels to provide insight into the relationship between God and the Children of Israel / the individual (anyone interested can read some of my thoughts on the subject here, though since then I've developed it a lot further). These include: Father/child, Husband/wife, Master/slave, and Shepherd/sheep. In different context different ones of these parallels are used. In Jewish prayer we find heavy usage of the Father/child and Master/slave parallels sometimes even juxtaposed like in the sixth blessing of the amidah.
However, the Husband/wife parallel is rarely used in prayer (perhaps the Friday night L'chah Dodi is an exception) and for good reason. It's completely out of place. A wife should never have to ask a husband for sustenance, clothes, healing, etc. these are all part of the husband's obligations! What kind of husband doesn't provide for his wife - that would be clear grounds for divorce (not to mention it seems like a horrible relationship). Which means, that to pose as God's wife and then ask for one's daily bread would be accusing God of not fulfilling His obligation - which seems like a pretty horrible thing to do.
2) I would just point out, that Judaism commands humanity to have children, be fruitful and multiply (Genesis 9:7), or 'He formed Earth for habitation' (Isaiah 45:18) and therefore the sexual act can not only be for the glory of God (which would be to strengthen the family unit) but also a (possibly) explicit part of fulfilling God's command.
