I was propositioned by my clients (fitness trainer)

I am a 38-year-old personal trainer.A little over a month ago, a man in his late 40s contacted me about training. He said he used to lift weights years ago but stopped because of work. He mentioned some shoulder problems and a desire to get strong again. We had a consultation call. He seemed straightforward. We scheduled an initial session and then a package of twelve sessions, two per week.During the second session, he asked if his 32-year-old wife could join some of the workouts. He explained that she had never lifted before and wanted to learn the fundamentals while training with him. I have a rate for duo sessions, so I gave him the price. He agreed and paid the difference.The third session happened with both of them present. We did standard exercises like goblet squats, hip thrusts, overhead presses, and some single-leg work. They both participated well and seemed to enjoy it.When the session ended, they were drinking water and chatting. The husband looked at me and said directly, “We really like your coaching style. You are relaxed, you explain things clearly, and you are our type. Do you ever do private sessions with the three of us? We are open-minded. We have done this before. There will be no awkwardness afterward.”His wife smiled and said, “There is no pressure at all. We want to keep training with you regardless.”I managed to respond that I appreciated their trust, but I needed to think about it and would prefer to keep things focused on fitness for now. They thanked me, sent a tip through Venmo, booked the next two sessions, and left. On the drive home, they sent me a full frontal nude of them both. Side by side. And another one of her naked ass. I have been thinking about it nonstop since then.

If I keep everything professional, I might lose them as clients once I clearly set the boundary. If I accept their offer, I cross a professional line I have never crossed before with paying clients. Either choice feels like a mistake at this point. What should I say to them at the next session? She's very sexy, but I am not bi.