While beef curtain quite clearly relates to the facilities of a lady to enable a gentleman to gain entry with his pork sword, the reason is hitherto explained. In the 1970s many members of the middle class had dreadful looking reddy-brown curtains hanging in their houses often smelling of the sunday roast which had been enjoyed by the said middle classes on previous sundays. More often than not these people would enjoy a good bit of beef which is why the said curtains smelled the way they did. A lady's receptical which enables penal entry has a very similar persona after many years of sexual attention as the aforementioned 1970s curtains. The expression 'beef curtain' found its origin in the late 1970s.
1970s reddy-brown curtains hanging in 1970s middle class homes smelling of the recent sunday roast often beef. Womans naughty bits look and smell the same after use.