Lost my virginity to my sister's horny male dog. Didn't even think him in a sexual way before it happened, but he mounted me one day while I was crossdressing in her clothes on all fours trying to get something out from under her bed. The short skirt and thong didn't stop him and he was able to get his cock inside my asshole and started fucking me hard. I had seen him licking his cock and it looked about the size of the applicator on my sister's tampons I had used anally before a few times. I was shocked at what was happening, but having his cock inside me fucking me felt good, so I stayed still and let him keep going. I had only seen him hump things for a bit and stop like he lost interest and he looked funny doing it. I didn't know about male dog's having a knot. When he stopped, I felt full like his cock had gotten bigger, but thought it was over when he released his tight grip on me, but it wasn't over. I didn't know it at the time, but he had gotten his knot into me and it had expanded inside me, locking us together. When he turned away from me, I felt something inside me turning as he did. When I tried to push his cock out of me, it wouldn't come out. He whined softly and started coming inside me. We weren't able to separate until he was finished, and in spite of my sister coming home early and finding us knotted and the horror of that, my insides were responding to his stimulation and his coming and then started contracting on their own, giving me an amazing feeling. It was wild, not anything I had planned on happening, and not the way I had planned on losing it, but that was the way I lost my virginity.
