I am 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 You Should Spy:

There are 4 important reasons you should find out if your spouse is cheating:

• To protect yourself emotionally. Worry over that nagging feeling in the pit of your stomach can be emotionally draining. Finding out for sure what is going on will enable you to stop worrying and start coping.

• To protect your health. If your spouse is cheating there is a danger of sexually transmitted diseases. If your spouse is having an affair and not using protection, they are putting your health at risk. Protecting your health is ultimately your responsibility and the main reason you should try and discover the truth.
• To protect yourself legally. If the infidelity means the end of your marriage some state divorce laws still allow it to be used as grounds for divorce. Even in states with no-fault divorce laws a judge can and may take into consideration the conduct of your spouse when determining spousal support.
• To protect your self-esteem.Infidelity in a marriage can cause a person to doubt their worth, doubt their attractiveness and question whether or not they themselves are responsible for the infidelity. It is imperative that you stand up for yourself if you feel your spouse is cheating. The best way to do that is gather information and confront them about their unacceptable behavior.

Reasons You Shouldn't Spy:

There are 3 reasons you shouldn't spy on your cheating spouse:

• You can handle the truth. If you are not ready, emotionally to deal with what you find, it isn't in your best interest to go spying. 
• You have a jealous nature. Every sign of cheating doesn't mean a spouse is cheating. Don't go off half-cocked every time you think your spouse is cheating. Especially if you are someone who constantly thinks their spouse is cheating. 
• If there is a possibility of domestic abuse. If your spouse has a history of abusing you, you should not put yourself in a position of being harmed due to their anger over being caught cheating. 

Whether or not to spy on a spouse who is cheating is a personal choice. If it is something you feel strongly about doing, for whatever reason then, only you can make the decision.