There is no question about it. Adultery is a serious sin in God’s eyes, and true worshippers must guard against marital infidelity.
لِذلِكَ عَلَى ٱلْعُبَّادِ ٱلْحَقِيقِيِّينَ أَنْ يَحْتَرِسُوا مِنَ ٱلْخِيَانَةِ ٱلزَّوْجِيَّةِ .‏