Disclaimer before I write this post: I have NOT cheated on my husband, and nor do I plan to.
So, going back many years, I lived with my ex fiance. I loved him lots, but I also cheated on him regularly - sometimes with different people, but at one point with the same man for about 18 months. I am not proud of my behaviour and have put it down to extreme stupidity of youth and general unpleasantness, which I have entirely outgrown and will never repeat.
I never once lied to him. I lied by omission, but I never told an outright lie. He knew who I was out with, but he thought he was a friend. The first time I cheated on him, I almost turned myself inside out with the guilt. But when he didn't find out about it, I did it again. We forget that real life isn't like a soap opera where infidelity builds to a ratings busting crescendo when the spouse finds out and all is revealed - it can carry on indefinitely, with people none the wiser. As people don't automatically assume that their partner is capable of infidelity, they often don't see what is obvious.
My husband also cheated on his ex with someone for a couple of years and had a similar experience to me - she still doesn't have a clue. We had a very honest and open conversation about it at the start of our relationship and have a pact that if one of us cheats on the other, then it spells the end of our marriage - no second chances, no excuses - it will be over. Because we've both been the cheater, we're more aware of the danger signs, and our pact keeps us from straying (well, it's kept me from straying anyway - I can only assume that he's been faithful, but the signs are good so far).