METRO.CO.UK - Dec 14 - The infidelity app Ashley Madison has threatened to sue a rival sex app after accusing it of copying its trademarks. It has ordered a hook-up app called CasualX to immediately change its branding and slogan or face legal action. CasualX, which markets itself as 'the Tinder for casual sex', advertises its services with the words 'Life is boring. Desire a fling?' Ashley Madison feels this is too similar to its own slogan: 'Life is short. Have an affair.' It also called on CasualX to stop using a picture showing a masked woman pressing a finger to her lips, which is allegedly similar to an image used by Ashley Madison.