This is what God says about adultery:
Exodus 20:14 Thou shalt not commit adultery.
Matthew 5:27-28 Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Hebrews 13:4 Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge.
Proverbs 6:32 But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul.
Ezekiel 16:32 But as a wife that committeth adultery, which taketh strangers instead of her husband!
Mark 10:11-12 And he saith unto them, Whosoever shall put away his wife, and marry another, committeth adultery against her. And if a woman shall put away her husband, and be married to another, she committeth adultery.
1 Corinhtians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
Ephesians 5:5 For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God.
Proverbs 5:3-8 For the lips of a strange woman drop as an honeycomb, and her mouth is smoother than oil: But her end is bitter as wormwood, sharp as a two-edged sword. Her feet go down to death; her steps take hold on hell. Lest thou shouldest ponder the path of life, her ways are moveable, that thou canst not know them. Hear me now therefore, O ye children, and depart not from the words of my mouth. Remove thy way far from her, and come not nigh the door of her house:
Proverbs 22:14 The mouth of strange women is a deep pit: he that is abhorred of the Lord shall fall therein.
Revelation 2:22 Behold, I will cast her into a bed, and them that commit adultery with her into great tribulation, except they repent of their deeds.
There are many other verses about this subject in God’s word. None of them have a good outcome for the soul of the adulterer. Temptation is hard to overcome but, even if you no longer love your spouse, you should still love God even more. Cheating on your spouse is cheating on God. You may be hiding in shadows or behind closed doors for fear that your spouse will catch you but, you’ve already been caught and His wrath is much worse than your spouse could ever dream up. It’s not too late though, stop and repent. You may end up losing your marriage, which you obviously don’t want anyways, but, at least you will be right with God. And if you want to keep your marriage then maybe, just maybe, your spouse will forgive you just as God will.
One good rule of thumb is, if you wouldn’t do it in front of your spouse, it’s cheating. If you have to hide it from your spouse because you know it will upset him/her, it’s cheating.