I've talked at length about Mr. Kay's feelings about spyware so let's detangle why this is bad, bad, bad....
1) A marriage is supposed to be about equality- When you start putting spyware on your spouses cell phone, computer, and even their car, you are tipping the balance. Whether you perceive that there's an affair or not is irrelevant. You should always communicate with your spouse any suspicions and ask for a proof of good faith if not completely convinced. But...do not think that they will show you. Your spouse still has a legal rights to privacy.
2) A marriage is not a parent/child relationship- Even if you put spyware on your child's phone, you should always tell them first. And only do so if you feel that there's a real perceived need, for example, a stalker or child predator or bully is targeting your child. Or if there's an over-abundant amount of evidence to distrust your child like traffic tickets, arrests, etc. But this is about your child towing the line. You cannot do the same to an adult regardless of the relationship.
3) A marriage is about trust- If you cannot trust your spouse, then you need to go to a counselor. Period. It matters not if the mistrust is real or not. You perceive it. Therefore there is something very very wrong.
4) Using spyware is abusive- Anderson Cooper (see videos below) interviewed two women whose husband's were using spyware at great lengths including video taping them using the toilet. As I explained in my first article about MMSL, it's an abusive measure. You should never, never, never have to use spyware unless...someone is bullying you, abusing you, etc. etc. and this is the most legal way to prove that. And in those cases only install it on your own phone or your own car. Meaning you install spyware in order to have your text exchanges copied. You should never use spyware as a means to manipulate a person, a power play, or uncover an affair. Nor should you put it on shared family computers or the other persons personal stuff (such as their cell phone). It's abusive.
5) Using spyware is, in some cases, illegal- It varies from state-to-state but putting spyware in areas where a person perceives privacy in most states is illegal. This includes tampering with private cell phones. (It's a bit different if it's a company phone.)
A Spy-Gear Arms Race Transforms Modern Divorce- New York Times (Oct. 6, 2012)
Danny Lee Hormann suspected his wife was having an affair. So the 46-year-old Minnesota man installed spying software on his wife's cellphone and the family computer, and stuck a GPS device to her car, letting him follow her to a lakeside cabin one night....The sleuthing got Mr. Hormann thrown in jail for 30 days, convicted of stalking his wife. "Whenever I tell people about this," Mr. Hormann said, "They say, 'I'd have done the same damn thing.' " He adds: "The technology just amazes me."