Good point about JA's naval connections. It also occurs to me that while Mary was raised in the household of an Admiral who was a man of "vicious conduct", Fanny was raised in the not very genteel family of a lowly Marines officer in Portsmouth - one of the biggest Naval ports in the country - so not exactly a sheltered upbringing.

You comment that Edmund isn't hearing the pun as a modern reader would also makes sense, and I'm beginning to think that the reason a shocked silence didn't descend over the dinner table is that Mary's pun simply wasn't heard as particularly shocking by any of the assembled company.

MP is set at a time when the robust earthy humour of the 18th century was only just beginning to give way to 'Victorian' primness. Political cartoons and other images from this period, produced by artists such as Cruikshank, Rowlandson, Gillray and many others, often contain extremely vivid sexual and scatological jokes. (I'm not talking about under-the-counter pornography here, though there was plenty of that too.) These images were freely available and openly displayed in print-shop windows.

So in a world when such imagery was commonplace, Mary's pun would not have seemed particularly coarse or remarkable.

This seems so obvious to me now that I can't think why I didn't make this connection before. Thanks for your input Bridget D - it was very helpful.

References:
"City of Laughter. Sex and Satire in 18th Century London." Vic Gattrell 2006.

"Decency and Disorder 1789-1837." Ben Wilson 2007.