This happened over a just over a month ago. My mother-in-law needed to have some household items fixed up for a party she was going to have a couple of weeks later. She lives a few hours away and my wife being eight months pregnant, I went alone.
I arrived on a Friday morning and starting working on the back yard. My mother-in-law was working that day as a substitute teacher. So I was able to get quite a bit done. After I finished with that, it was early afternoon and I was hot, tired and extremely sweaty. I decided to take a shower. The shower in the second bathroom wasn't working so I used the shower in the master suite. The shower there has glass doors and walls. None of which is frosted. Also, there is no door between the shower area and the bedroom. Anybody walking into the bedroom could see into the shower and see everything quite clearly. The thought of someone walking in on me got me horny. When I got in the shower I started to stroke my dick using the soap as a lubricant. I made myself stiff and started to play with my anus. I really enjoy anal stimulation but I've had a hard time in the past trying to tell my wife that I like it. I stuck my finger into my ass and stroked myself harder. It felt good but I wanted deeper penetration than I could get with my fingers. I then remembered that in my mother-in-law's kitchen was a large spoon with a round handle that would do nicely. I quickly ran naked and wet to the kitchen found the spoon and headed back to the shower. I soaped up the handle and slid into my ass. It felt great. With one hand I pumped my ass with the spoon and the other I stroked myself. I did this for several minutes. It felt so good that I wasn't aware of my surroundings. I let the pressure build are far as it could go and then let loose with a great orgasm shooting my load all over the shower walls. As I came down from my orgasm, I heard a noise and found my mother-in-law standing just a few feet away from the shower. I was completely mortified. But she only smiled and turned and left the room to let me finish my shower in peace.
After the shower I came out to the kitchen to talk about what had happened. She said I shouldn't be too embarrassed and said she wouldn't tell anyone. And then admitted that she had just come home when she spotted me going to the kitchen naked with an erection and then watched me take the spoon back the bathroom. She quietly followed me to the bedroom making sure I didn't notice her and watched me get myself off. She hadn't seen a guy play with himself in a long time, so she enjoyed the show.
Needless to say the next two days when my mother-in-law laid out the towels for my shower she laid the spoon on top of them. I invited her to watch, which she did. It was a good weekend.