It is the sin of immorality. A number of Christians have fallen into that pit by letting themselves get into situations in which compromise is all too easy.
وَأَحَدُ ٱلْمُغْرِيَاتِ ٱلْأَكْثَرِ فَعَّالِيَّةً ٱلَّتِي يَسْتَخْدِمُهَا ٱلشَّيْطَانُ يُشْبِهُ هذَا ٱلنَّوْعَ مِنَ ٱلْفِخَاخِ ،‏ أَلَا وَهُوَ خَطِيَّةُ ٱلْفَسَادِ ٱلْأَدَبِيِّ .‏