Friend's Underwear

I'm not gay, but the thought of other guys masturbating has always been a turn on for me. I was particularly curious about whether my best friend, who is very good looking, did it, and one night this winter I found out. I was sleeping over at his house, and fell asleep, but was woken by noises in the other bed. I looked, and realized my friend was humping his pillow. He must have thought I was sound asleep, because he had his eyes closed in concentration, his covers were off, but he still had his underwear on (tighty whities). I was fascinated watching him, and put my hand into my underwear to rub my own penis. When he came, he shot into his underwear and just turned over, pulled up his covers, and went to sleep. I let loose at the same time he was doing that, so he didn't notice any movement from my bed. The next morning he got up before I did, and I saw he had a piss-hard, and I could see a wet stain in the front of his underwear. He grabbed a clean pair from his dresser and went to the bathroom. Later I went to the bathroom and felt around in the clothes hamper and found his still-wet underwear. I stuffed them in my pocket and took them home when I went, and masturbated while looking at them and smelling the fresh semen smell. I felt a little guilty about that, but it was just so exciting to know he did that, and to be able to feel his wet semen, that I kept them and used them like that many times. A couple of other times I've seen him do that, and I've snitched his underwear again, returning the older ones the next time I went to his house. I still have a pair well hidden, and though they are dry, the crusty stains are still very obvious. I would be mortified if he found out, but I like it.