fornication       
noun [ U ]  formal disapproving uk/ˌfɔː.nɪˈkeɪ.ʃən/ us/ˌfɔːr.nəˈkeɪ.ʃən/
 私通,通奸
the action of having sex with someone who you are not married to
  Adultery is a graver sin than simple fornication. 婚内通奸是一种比简单的私通更严重的罪。 
 He had no thoughts of fornication in his head. 他脑子里没有通奸的想法。 
 See: fornicate
 - More examples
 - The drama is full of scenes of fornication and betrayal.
 - I stopped consciously thinking about fornication the moment I entered Sheila's flat.
 - If she commits fornication, then she has broken the marriage vow.