My Secret Fantasy with a Stranger at Workouts

I've been in a steady relationship for two years now and everything is great between my partner and me. We support each other, we have fun together, and I genuinely care about her. But there's this dark little secret that’s been gnawing at me lately, during my personal training sessions with clients, especially those who are new and unsure of themselves, I've had these strange fantasies. When they're trying to lift a heavy weight or push through the last set of an exercise, sometimes I feel this overwhelming urge to help them beyond just coaching them on form. It's not like anything actually happens, but there's a sense of wanting to get closer to them in ways that make me squirm when I think about it too much. The feeling is so intense that it’s hard for me to shake off and focus. Is this normal? Should I be worried?