I had been married to my wife for just on 10 years. She had come from one of those, "no sex before marriage-type" families but I can attest that she did not follow this while we were dating and during our engagement.
My wife and her mother were from European stock, which made them both attractive and passionate about things they cared about. Her mother had long divorced her husband so from the time I knew her, it was just her.
My mother-in-law was one of those that made you feel very welcome and loved to talk to you endlessly, in a very genuine kind of way. Once when I was training for a marathon she would come around quite often during the week. I would get home from work, give her a quick kiss on the cheek and try to get ready to squeeze in a run on my home treadmill. In my rushing about, I would start to strip off in front of her as I gathered my water bottle and towel. I often got down to my underwear as I quickly moved around her, wanting to knock out my run before dinner.
After a quick (30 minute) run I would be a mass of sweat and then have to take off my clothes to jump into the shower. I would start to strip down all the way to my underwear, my body glistening with sweat and my mother-in-law seemingly wanting to start a new conversation about something. I could never quite bring myself to strip off my underwear right in front of her, although the thought did go through my mind to do this many times. The chair she usually sat in gave her a view down out hallway. Once as I went off for my post-run shower, half way down the hall I pulled down my underwear. Whether she saw anything or not, I didn't know. I thought of walking back the other way as if I had forgotten something but it wasn't something I had the courage to do.
One Xmas we stayed at her place which was a three hour drive down the coast. She had a treadmill, so I was able to keep up my running during the time we were there. Being Australia, Xmas time is summer and it was very hot in the garage where she kept her treadmill. I was sweating in no time. For some reason I was quite horny after my run, so I stripped off all my clothes, including my underwear. My wife had gone for a dip in the pool when I had started and I knew she would be there. I knew also that my mother-in-law would be in the lounge room sitting on her chair. I bundled up my clothes and pressed them to my groin as I went from garage to the house. As I passed through the lounge room I froze when I saw her. "I thought you'd gone swimming", I said. She indicated that she wanted to have a rest. I didn't know what to do next. I couldn't very well drop my gear onto the floor. We made a bit of small talk and then I went off to have a shower.
A bit of a breakthrough came when all three of us went swimming a few days later. I'm not one for swimming so it surprised no one when I announced after about 20 minutes that I'd had enough. My wife was at the end of the pool I got out of and her mother was half way down the other end (so, about 10 metres away).
I grabbed my towel and began drying off. I started to peel my swimmers off as I continued to dry. I had no underwear on, so I was able to dry all over and flash my dick as I did it. My wife could not see what I was doing as I was standing right above her. Even if she looked up, I would be able to cover up in time. My mother-in-law however could see me as she was looking in our general direction. I didn't push my luck too much and after pretty much being dried off I went back in the house.
I would find out later from my wife that as much as I had been trying to show my dick to my mother-in-law, she had been equally keen to see it. Early on in our marriage my wife had apparently said to her, "If a boy shows you his willie, he trusts you. If his willie gets hard, he likes you". She had taken this to heart and, being her new son-in-law, wanted me to both trust and like her.
It may have been the holiday season, or summer or simply the fact that we had known each other for 10 years but my mother-in-law decided to progress things. Traditionally at Xmas we would give each other presents but they were always modest (think pair of socks or a book).
Xmas day came around and we had a nice dinner. We were all drinking a little bit more than we had been and that evening we came to exchanging presents. I received a small parcel from my mother-in-law and immediately my thought went to socks or a tie. I opened it up and it was underwear. Very good quality, silk underwear. Again, I wasn't aware of it at the time but my wife and mother-in-law had discussed this and thought it was a good idea.