I've been a member of a well known gym in our area for many years. My wife has never really been interested in going to the gym, but to be honest she is really slim so doesn't really need to. A few months ago I was quite shocked that she wanted to become a member and said she just wanted to get fit, having rarely exercised. She signed up with a personal trainer who she had sessions with a few times a week. I was at the gym whilst she was having one of her sessions and kept looking over to her to see how she was getting on. I noticed her pt was very hands on and could see him touching her lower back and occasionally her bum. I was feeling jealous but also very turned on and could feel my cock stiffening. I left git a shower and waited for her to finish. That evening when we where in bed I brought it up how close her pt was with her. She just laughed it off. I was getting aroused talking about it and asked her if she got turned on when he touched her. My wife is quite straight so I was really surprised when she she said yes, and that he makes her really wet. At this point I was busting and cum inside her at the thought of it. The following day we were at the gym and my wife was wearing her normal kit but without a bra. She has nice big nipples and I could tell her pt had noticed this. The gym was quiet and although he knew I was training across the other side, they were clearly flirting with each other. When we had finished my wife told me that she had asked him to call to the house for her next session. I was a bit unsure about this but still turned on at the thought of it. I left for work in the morning and my wife was up and dressed in her gym gear. Her pt was calling at 9 so I decided to call back at the house to see what was happening. I crept around the back of the house and peered through the window making sure I wasn't seen. My wife was bent over a inflatable ball and struggling to kept her balance. At this point his hands were all over her waist and arse. He steadied her and continued to caress her buttocks. I knew she was enjoying this and she then rolled forward placed her hands in the floor and kept her arse in the air. I watched him peel her Lycra pants down, revealing a black g string. My cock hardened and I was mad but so turned on. He continued to rub her arse and slipped his hand between her legs. She was now moving her arse faster as he rubbed her wet clit. I released my bulging cock from my pants and starting wanking. He took his shorts off and started rubbing his hard cock over her cheeks. He then slid his cock inside her and started fucking her really hard and fast. After a few minutes he pulled his cock out and she came off the ball and dropped to her knees. She took his length into her mouth and was rubbing his balls with her hand. I exploded at the sight of this and zipped my pants up and went back to my car. I waited outside the house for almost an hour before he left. I then left the car and went into my house. My wife was in the shower when I went upstairs. I asked her how her training session went and she replied "good, I'm a bit sore though, he worked me hard". We had sex later that day and I asked her if anything happened, she told me to stop being daft and it's purely training. She is such a slut as I watched her being fucked and sucking his cock. I was turned on though and she is having another home session next week.