I'm sorry to say that I think it is another example of the (some would say hypocritical) way the Catholic chuch thinks... Another example is that, if you are to pay the Curia nicely, there are more reasons for which a marriage can be annulled in the catholic thinking than there are excuses for a divorce, but you are not allowed to divorce... and incidentally any children you had before a marriage was anulled were born within wedlock despite the annulment meaning that the marriage never existed in the first place!
As you say, sex is one of the things whioch helps to bind a marriage togerther, - it is not (at least IMO!) purely for the purpose of procreation, but is an expression of the mutual bond. The trouble is that the RC have got thoroughly hung up on sex to an even greater extent than the C of E - which is saying quite something!