I presume Rita Mae means that riding side saddle would protect mens private parts better? English aristocrats are often stereotyped as being peculiar, so she is being sarcastic here. Scunthorpe is not strictly a tmesis. Many tmesi involve swear words, so i'm not sure i should be explaining this on a public forum as the swear word inside this word is probably the most offensive swear word in the English language. PM me if you really want to understand it!