Each of us (chooses) our friends, lovers, and spouse. If this is a recurring theme in your life you may need to examine your criteria for selecting people to become involved with. You get to choose who you spend your time with. You're responsible for your happiness.
Trust like respect is something that should be (earned) overtime. It's possible you have been trusting people before allowing them to demonstrate they are worthy of your trust!
"Never separate your mind from your heart when making relationship decisions. The purpose of the mind is to protect the heart."
Whenever people choose to ignore "red flags" or don't ask questions and analyze the answers given to them they're bound to see and believe whatever it is (they want) to see and believe.
Lastly there are two reasons why people lie for the most part.
1. To avoid pain (Either their own from your anger, punishment/breakup, or they don' want to hurt you with the truth/disappointment)
2. To keep the thrill alive! (They don't want to be asked to stop doing whatever it is they have been doing behind your back.)
The goal of a cheater is to hold onto all that is good in their primary relationship while addressing their other wants/needs on the side. They're not looking to give up one relationship for another.
Cheaters don't expect to get caught!