This is a short story but it does have to do with masturbation.
I have really oily skin, it's been that way since I was 12 so now I am in the habit of wiping my face off with a towel whenever I feel it getting really oily rather than washing my face a bunch of times every day. My Dad's girlfriend has a 14 year old son who jokes a lot about masturbating and sex. The other day my face was real oily so when I came home I went to my room and grabbed one of the towels I use but when I wiped my face the middle of the towel was hard and had a strong odor of semen. It was a dark blue towel and I could see that there was indeed some type of stain. I was more amused than I was grossed out. Later that day I asked him if he had jacked off with my towel and he started laughing and asked if I had used it, when I told him that I did use the towel and what it smelled like and felt like he just got this really slick grin on his face. He now owns that particular dark blue towel.