I had the luck of growing up in a farm. Had plenty of animals around, and I can even tell you how exactly I started. Once I discovered masturbation around 12 or 13, I was doing it at least couple times a day. But being shy and awkward, I would never dare to start dating.
One late autumn day, I was cleaning out manure. Nobody was home. One of the cows started urinating. I stared at the bulging vulva for a moment, and suddenly the most exciting realization came to me - I could have sex with the cow!
I was shaking out of thrill while setting up the wheelbarrow to access her backside; she was a fully grown cow and I was just a wee lad.
Luckily for me, she was one of the easy going ones. And my cock wasn't that big, either, so it was no issue to slip it in the vagina. Gosh, I still remember the texture of her warm inside. That was the best sensation I had ever had. My first girlfriend 10 years later was no match, I must say.
Occasionally I would do it with some of our pigs. Their pussies were wonderfully tight and soft, with that protruding lip touching my balls.
Unfortunately, since I have moved away from home and live in the UK, I haven't had any chance to mate with animals. I'd love to, even though I have a fiancee now and a fairly standard life.
Anyway - to OP, the taboo only exists in the society. People who have been raised outside mainstream, will not consider it a taboo. And different societies will not consider it a taboo, either.