They have different sexual natures. The Saturn ruled Capricorn will be at odds with the Mercury ruled Gemini. The Gemini are usually easy-going, amorous people, while the Capricorn are sluggish, devoted, cautious. Capricorn will have a hard time holding the reins on excitable, flirtatious, wayward Gemini. All of Capricorn's patience is needed to deal with a partner it considers to be capricious, extravagant, and scatterbrained. In order for the sexual relationship to be a success, Gemini will have to distract Capricorns mind from his/her career and the problems that go with it.
The initial interest can not last very long.