I’m not sure ethics has anything to do with whether or not you should spy on a cheating spouse. If your spouse is cheating, it’s my opinion that a person has to do what needs to be done to protect themselves. As long as your motives are not to get revenge on a cheating spouse or to use the information you find to cause problems for the other man/other woman, I see nothing unethical about spying.
Things to Consider Before Spying:There are things you should take into consideration before you begin your quest to find out if your spouse is cheating. First, are you ready for the conflict that will occur when your spouse finds out you have been spying on him/her? You should expect your spouse to be upset, to accuse you of not trusting them and to deny their own actions in favor of trying to make you feel guilty for spying.
Secondly, certain forms of “spying” are illegal. There are state and federal laws that regulate the use of surveillance equipment, recording devices and Internet software. You should become familiar with your state’s laws before you go full steam ahead with any sort of covert spying behavior.
Reasons Why You Should Spy:
There are 4 important reasons you should find out if your spouse is cheating:

