My mother in law mentioned she needed to get a new swimsuit and when I mentioned I needed a new one too she mentioned we go out and shop together. She said she knew a place and that is where we went. It was just us and not my wife. She quickly found one for herself and now it was my time to look for one. I was tried on a few and it was the speedo types I liked the most so it was narrowed down to a few of them. There is one I liked the look of but if felt a little too snuggle on me. The saleswoman helping us said it looked like it was my right size and thought that the underwear I was wearing underneath it was causing it to feel snug. I tried the next size up and it felt better but the saleswoman said if I was without my underwear it might be too loose and mentioned that water would also loosen up the suit on me. They made you keep your underwear on when trying on suits for sanitary reasons. My mother in law asked if I could try the swimsuits on without my underwear to know better about the fit. The saleswoman was a little hesitant at first but ssid that I could quickly try them without my underwear to get a better feel on the real fit of the swimsuits. I tried the smaller one first since the saleswoman thought it was going to be a better fit for me. I came out in just the suit and the saleswoman put her fingers in the waistband and pulled out to show how it was a good fit and not too big on me. My mother in law did the exact same thing to see for herself and said it was probably the right size but to try the other just to compare. I put the other suit on and immediately my mother in law stood in front of me and put her fingers in the waistband and pulled out to see the fit. When she pulled out the waistband on this suit it was looser and came out further which exposed my cock to her and the saleswoman. She was pulling out the waistband and tugging on it to see how far it would go out. I was in shock and the saleswoman didn't seem to know what to do so she just acted casually. My mother in law said she could tell they were a little large on me and said I wouldn't want them falling off unexpectly if I went swimming. She came behind me and pulled on the waistband from the back and basically was exposing my ass now. She then had me put the first suit on again and did the same thing with the waistband but this one didn't show me off much and she said it was the better fit. She then had me try on the second suit again and pulled out the waistband once again showing off the front and back side of me once again. We bought our suits and left. On the way home I mentioned her pulling out the waistband and exposing me. Her comment was that if I had issues with it I should have said something when she was doing it not after the fact. I told her it caught me off guard and I was in shock. She said it was nothing to be concerned or embarrassed about and that she has seen guys before and the saleswoman mostly likely has too. I asked if it was appropriate for a mother in law to see her son in law like that. She said maybe not but she has now so I had better just deal with it. She added she had wondered what I was looked like but since she has seen me now she didn't need to wonder any longer and said it was nice change from how she normally sees me.