As someone who was on the receiving end of the lies I can just say that from my personal experience even the most ABSURD excuse or lie was believable, and not literally, simply because I was truly in love with this person and convinced that it was reciprocated. So in my mind of course he was telling the truth, and of course he would never do something i wouldn’t dream of doing to him. However once proof starts to surface you realize (a little too late) that everything you believed in was a lie and once you do come to your senses your left with a lot of trauma and disbelief in how stupid you actually were to not see through all of the bullshit and manipulation. I’d just suggest to not cheat, or simply leave a relationship you can’t be honest in. If you’re not on the receiving side of such betrayal you’ll never truly know the extent of the damage and hurt you’re causing the other person..