Naked grappling

My friend and I both were interested in martial arts and joined a grappling martial art. The instructor had a policy that when a student got to be a brown belt, they would have the key to the studio and were allowed to practice anytime they wanted. So my friend and I started to go over to the studio late in the evenings to practice when nobody else was there. My friend heard that the best way to learn how to fight is to fight in the nude, as a man feels most vulnerable when naked, and you gain more confidence in your skills when you see what you opponents body. My friend and I have by this time also for years been masturbating and having sex with girls for years and were very comfortable grappling in the nude so much so that after a while to keep things interesting we often times brought a blow up doll and had sparing matches and games to determine who could cum in it. There were several variations of the sparing match games that we would do. One was we would grapple with one another until one of us was so exhausted the other would start humping the doll trying to cum before the other would get their wind back and we would start grappling over again. Another game that we played was if one got a groin strike in the one who had the groin strike would count from 1 to 30 and the other had free reign trying to cum in the doll. A joint lock that would lead to a break, you got 60 seconds. One game that I really liked was using simulated weapons like a rubber knife or bamboo swords, we would write on a pieces of paper times from 30 seconds to 5 minutes in 30 second increments and one of us would randomly draw a time and would go into the change room for the allotted time, while the other would start humping the doll facing away from the locker room. The goal was for the person in the locker room to strike to one humping the doll with either the rubber knife or bamboo sword. The one humping the doll goal was either to cum in the doll or if he felt the other approach to defend themselves. If someone got hit with the rubber knife or sword, they would randomly draw a time and wait in the changing room. If the person humping to doll got hit, then the attacker got to hump the doll, while the other guy would randomly draw a time to stay in the changing room, and the game would go on. One time we were playing this game, when I snuck up to my friend, he sensed me and we grappled with the rubber knife. I got finally got on top of him and he wrapped his legs around my waist. I could feel his hard cock against mine. I had the rubber knife above his chest trying to get him with it and he was holding my hands trying to stop me. My weight gave me an advantage and I slowly brought the rubber knife down on his chest. Just as I tagged him he let out a startled grunt and started to shake. I thought I actually hurt him, till I felt his cock start to twitch and I felt his hot cum shoot against my rock hard cock. I was very close to cumming, when I tagged him and the feel of his cum shooting on my cock made me cross over and seconds later I shot my load over him. We only stopped these games when we almost got caught by the instructor. We heard him come in one night while we were naked in the changing room with the blow up doll. We managed to hid the doll and put on pants just as he came into the changing room. After that we never dared grapple naked there again.