Never Thought I Was A Cuckold Until..

Never even knew the word cuckold when this happened. Early twenties my gf and I were going strong. A friend of mine I've known fir years met us for a drink. Busy bar, loud. Us 3 were sitting at a table gf an me on one side him facing us. I excused myself to go to the toilet. When I came back we'll, he had changed seats and sitting next to her and talking quite close every now and then he would touch her hand. I froze looking from a distance, not knowing what to do. I just watched. Yes I should have gone over. But that's when I felt the tingling in my groin. I realised I was getting semi, precum leaking. I was so embarrassed at this went back to the toilet to relieve myself. Came back and both were know sitting opposite. I was flustered but we carried on our evening. 3 or 4 times my friend was playfully touching my gfs hand, she never pulled away. We got up he held her waist as we moved through the bar me following. It should have been me holding her waist, but I just felt like I couldn't stop him. We both went our separate ways he home us 2 to our apartment. From then on I could only think about her with him or someone else. Came very quickly that night. We broke up within a month, she saying things not working out. Been having these feelings for life now and John my friend had planted the seed