This was when we were both 20. We had been each other's first and had been living together for a few months. I've never been super into anal and we'd never tried it before but had had a pretty bad day and was feeling pretty agitated and needed to blow off some steam; so when I got into bed I began to feel her up but she had been really bitchy all week and wasn't really into it as she was already mostly asleep. Eventually she relented after I pulled my cock out and was rubbing myself between her legs, she mumbled that was fine and I could get off that way "just be sure to cum in a paper towel or something" yea right. I slipped her panties aside and forced myself in bit by bit until she loosened up and began to get wet. This pissed me off because here she had been saying she didn't' want it and had been a total cunt all week yet here she was getting wet and moaning while thrusting back against me after just saying how she wasn't in the mood. So I decided if she wanted to get fucked then I would oblige, and I flipped her onto her stomach and started to go harder and faster. At some point I slipped out and as I hastily forced myself back in it went into her ass instead by accident while lubed only by her own juices (I should mention here that I'm fairly well endowed so this must have been rather shocking). She immediately began to squeal and squirm but I already had my arm around her neck and was halfway into that virgin ass with my other hand on her mouth. I immediately realized what had happened but instead of pulling out I forced myself the rest of the way in squeezing myself up to the hilt till my balls rested against her still leaking pussy. Up till this point it had been mostly instinct just forcing myself into this tight warm hole but now fully inserted with her whimpering, and shaking underneath me I was overcome with a sense of satisfaction at her pain, and a feeling that she deserved to suffer for being such a bitch recently. So I pulled myself all the way out till just my tip was pressed against her ass again and as I did I felt her let out a breath she had been holding and begin to sob. Then I forced myself back in again this time much slower, then faster and harder until eventually she just lay there no longer sobbing or squealing; just taking it until I eventually pressed deeper than ever before and unloaded one of the biggest loads of my life that had been pent up all week into her ass. After a bit I rolled over and pulled her back into the spooning position we had begun in with my half chub cock still in her ass, and slept like a baby. When I woke up she was in the shower so I went to join her and told her how tight and satisfying her pussy was to make her think I hadn't realized what I was doing. She blew me in the shower and we went about as normal, I never did fuck her like that again and a year or so later we broke up for unrelated mutual reasons.

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I have one other story like this of my best friends ex that I might share if I feel like it sometime, let me know if you'd like to hear it!