I did this to a dude once. We had played a few times and he always insisted I wore a bag. This one time I was fucking him pretty rough - he was on all fours and I had mounted him from behind. I had a bag on as I slammed his hole. I worked my arms under his and then locked him in a full nelson hold. Then I really went to town. I let him realize that he was in a lock hold and couldn't escape and while I fucked him I told him that the bag had broken and that this time he was going to take my load. He began to really struggle to break out of the hold but he couldn't and all the struggling put me over the edge. I popped my load while he is screaming about safety and shit. I remember the last thing he said while I was still in his ass was asking me if I was sure I was neg. I replied "Too late. Whatever I got, it's inside you now." I pulled out and he was freaking out and then I showed him that the bag was still intact. He emailed me for months after that telling me that it was the hottest fuck he had ever had.