Conflicted Desire During Training Sessions

I'm a personal trainer and have been married for five years. My wife knows I enjoy my job but doesn't know about the secret desires that come with it. A few months ago, while training a new client, Lisa, things started to get complicated in ways I never expected. Lisa is beautiful, athletic build, toned muscles, and a vibrant energy that matches her red hair. She's always enthusiastic during our sessions and even when we're not working out together, she keeps up with me on social media. That connection turned into something more for me as the weeks went by. I started craving physical closeness with her. It all began innocently enough, small touches that should have been part of a typical training session but felt charged with an undercurrent of lust and desire. As we progressed, those casual brushes became more deliberate, and I caught myself staring at parts of Lisa's body that made my heart race faster than any workout could. One day, while helping her stretch out knots in her back muscles, I couldn't stop imagining her letting go and leaning into me deeper. My hands started to wander beyond what was necessary for the exercise, exploring areas where only lotion covered her skin. Lisa never said anything about this being inappropriate; she didn’t push boundaries or complain. In fact, some of our sessions felt like a mutual exploration of boundaries that neither of us explicitly defined. Yet, I'm terrified of crossing lines that are there but unspoken, especially considering my marriage. How can it be normal to feel this way towards someone who's not my spouse? Should I confess to Lisa and risk damaging both professional relationships or continue hiding behind the guise of a personal trainer? Am I an asshole for keeping these thoughts secret from my wife while being so open with Lisa about everything else?