My first time getting fucked scared me. I was spending a few days at a friend's house for the Christmas holiday, and his (intact) white pit bull had been hitting on me the whole time, showing me the tip of his cock, trying to mount me, nuzzling my crotch, etc. My friend had to go do something with his boyfriend for several hours, and they left me alone with their beautiful, randy pooch. As they left they made a joking comment about how I had my "boyfriend" there to keep me company. I never confirmed it, but I suspect that they were having a polyamorous relationship with that dog.
Anyway, as soon as they left I stripped naked and started frolicking on the floor with the dog. When he started trying to mount, I got on all fours and turned my rear to him. I hadn't lubed or prepped because, In my virgin naivete, I figured I could just let him rub his cock between my ass cheeks, see how it felt, and figure out what I wanted to do from there. I knew nothing about canine anatomy or knots. This was in the early days of the internet, before I had discovered online porn or zoo forums.
That pit bull was a pro. With impeccable aim, he slipped that thing right straight up my ass on the very first stroke. I didn't even realize I'd been penetrated at first, but then I felt a throbbing pressure inside my ass and up against my prostate. I felt a sharp pain but, along with it I was taken by an exquisite sensation that made me let out an involuntary moan. I was genuinely befuddled about what was going on back there, and, like an idiot, instead of just going along for the ride and enjoying it I reached back to feel around. The moment my fingers brushed up against the dog's nuts, he yanked himself out of me with a loud pop. I felt a sharp stab, like something had just torn in me, and I felt liquid gushing out, and when I turned to look there was blood mixed in with the puddle on the floor that the dog was lapping up. I ran to the bathroom and got into the shower, terrified at the thought that the blood running down my legs meant that I was going to have to go to the emergency room. I furiously racked my brain for some plausible-sounding cover story, but after 15 minutes the bleeding stopped. I recovered OK, but sitting was painful for the next week.
Unfortunately, the circumstances of my life have prevented me from having access to a male K9 to mate with, so this was not just my first time, but it has so far been my only time getting fucked by a dog.