Actually, some of us do. For instance, those of us who try to follow Christ also, by definition, try to give at least some consideration to how Jesus defined it:
"27 You have heard that it was said to those of old,[c] 'You shall not commit adultery.' 28 But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart."
(Matt. 5:27-28, NKJV)
The lesson: Big sins usually start off as small ones. Don't lust after other women, don't think about cheating, don't put yourself in the position where you might, don't neglect your marriage to the point where you feel you need to, and chances are pretty good that you won't.
We also try to remember that if God has forgiven us our sins against Him, then we also ought to be willing to forgive those who sin against us. Most marriages can be saved if both partners are willing to save it, and, sometimes, even if one of them is not.
The bible says it's okay for your daughters to get you drunk and have sex with you also.