I was... younger than I am now. Compared to other people, a very normal age for anything of a sexual nature.
I'm not completely sure of how or when, but I think it was partly due to the tv shows my parents watched. Often times about animals, like veterinarian shows or just animals doing whatever for the show. I saw things like horse penises, pigs with massive testicles, even a cow getting fisted (checking for pregnancy, not sexual but I didn't know at the time).
So, normal, natural things. It was interesting, but strangely more than just "interesting". Of course, being horny, I looked at normal porn. At some point those two managed to collide.