Secrets of a Super Stud, also known as It's Getting Harder All the Time and Naughty Girls on the Loose, is a 1976 British sex comedy film, one of many to be also filmed in a hardcore version for export. It was shot at Twickenham Film Studios under the title "Custer’s Thirteen", the general release prints and the hardcore version were processed at Kay Labs at Highbury, North London.

Plot

Custer Firkinshaw (Anthony Kenyon) the owner of 'Bare Monthly' Magazine is up to his neck in dirty pictures and sexy secretaries. His hedonistic ways are however temporary halted when his Uncle Charlie dies and Custer is pitted against his relatives. His Uncle leaves Custer a fortune in the will, but only on the condition that he marries and has a child within 12 months, otherwise it all goes to his relatives. Custer’s money grabbing Aunt Sophie (Margaret Burton) knows only too well about Custer's swinging ways, so keep tabs on him by hiring a crooked private detective Bernie Selby (Alan Selwyn). When Custer visits a doctor both parties discover that due to Custer's oversexed lifestyle he's only got 13 units of 'sexually activity' left, meaning he has only 13 more attempts to father a child.