I never expected anything like what happened to happen to me either. Before it happened, I knew nothing about dog sex and didn't see dogs in a sexual way at all. My sister had a horny male dog that humped things a lot, but he didn't seem to last long and would stop like he had lost interest, and I always thought he always seemed silly doing it. The day it happened to me, I was in my sister's bedroom crossdressing in her clothes on the bottom wearing her short skirt, thong panties (first time trying that day), nylon socks, and heels. I didn't see my sister in a sexual way either, but I loved dressing in her clothes. I had gotten down on all fours to get something out from under her bed when her dog mounted me, gripped me tightly, and started humping. I didn't know what he was trying to do or what I was in for, but the thong panties and short skirt offered no protection and he was soon inside my asshole fucking me hard. I froze, I think I from the shock of what was happening and didn't move at first. When the shock wore off some, what he was doing started feeling good, so I let him keep going. I didn't know about male dogs having knot on their cock. I thought he'd hump for a bit, then stop. When he finally stopped and his grip softened, my asshole felt full, but I thought he was done and would get off of me and head off somewhere. I found out I was wrong when he stepped off of my back and turned away from me and I felt his cock still inside me turning with him. We ended up ass to ass stuck together. I tried to push his cock out of me, but it was stuck in me somehow and wouldn't come out. He whined softly and I felt strange inside knowing that he must be coming in me and there was nothing I could do to stop it or get away. I felt helpless. It was very embarrassing and humiliating as my sister came home early that day and found me wearing her clothes and ass to ass stuck together shortly after he had turned. Having his cock and knot stuck inside me pulsing and throbbing, I felt sensations I had never had before. Before it was over, my insides starting contracting and my asshole clamped down on his cock and I moaned in pleasure from it.
After losing my virginity to my sister's horny male dog, I told myself nothing like that would ever happen again and even blamed myself for it happening, the position I was in, not knowing about how dogs fucked, and for what I had been wearing. I found myself looking at his maleness a lot and thinking about what happened, being helplessly tied to him by his knot, and the pleasure I had felt from being his bitch. To my surprise, I started getting urges to let him fuck me again. I tried to fight those urges, but after that first time getting fucked by her dog and getting pleasure from it, I soon horny and wanting to get fucked again. I dressed in some clothes my sister had given me and called him to my room. I locked the door and quickly got down on all fours flipping my skirt over my back and pulling my panties down. Her dog seemed to know what I wanted him to do, and after licking my asshole, which felt amazing, he mounted me and fucked me again. The sex was much better the second time, since I wasn't trying to get away from him and I moaned loudly when the pleasure came again. I was completely hooked on dog sex. I don't think my sister realized her dog was fucking me almost daily, but if she did, she never said anything.