unfaithful
adjective uk/ʌnˈfeɪθ.fəl/ us/ʌnˈfeɪθ.fəl/
不忠的，通奸的
having a sexual relationship or experience with a person who is not your husband, wife, or usual sexual partner
If a man was unfaithful to me I'd leave him no matter what the circumstances. 如果一个男人对我不忠，无论如何我都会离开他。