In my case i think it's a mixture of everything.
1- It IS very sexual, as i'm into it doing myself and also looking at who would be my partner into doing it, the anatomy, temperatures, etc, it's a big turn on to me, also i'm really curious to participate into a three-four-some with a girl and dogs, which i can't achieve, sadly.
2-It's a bit of instinctive as well, i mean, i've always been more closer to animals than people, this doesn't mean i don't like people, but i.e , if i visit someone's house, i tend to pay attention to the animals within the house before the people living in it, not in a sexual way, but their presences are kind of more determined to me than people's, also i have big empathy for animals and i feel the whole sex seems so natural, so pure, between two beings, i don't say humans can't perform the same type of sex, but with an animal you'll experience it pure.
3- The thrill, oh god the thrill, the idea of doing "something naughty", the whole idea of performing something "taboo", i mean, i know my reasons to consider that what i like "isn't bad" but hey, i'm no one to say what's good and what's bad, i always said that as long as everyone enjoy it and nobody gets hurt, then, why not ? but yeah, my mind blows when i'm about to do it, or when i was about to do it. It adds to the whole idea of "having sex" which i see it as something naughty, but intimate and closer at the same time, a good combo.
4-And about meaningful ? yeah, i mean, this is something i learnt at the last, that dogs actually enjoy it , and this was what motivated me to move on and finally try it. Since i had my stages, and then after seeing how keen and energic they are to perform it again, and the way the react to you if you've done everything nicely, it's pure love, not in a romantic way, but i love animals, and i love being loved and to love, and when i feel an animal's interest into nuzzling my arm, or just wanting to be close to me since they enjoy it, it weights lots to me in an emotional way. This plus the fact they enjoy sex, or at least that's what i believe, since i don't think they'd just act by repetitive behavior, and i saw with my own eyes a dog coming back "for more" without me showing any interest, it was that moment when i realized they enjoy this and how could i say no to that ? i mean, i don't hurt them, they don't hurt me (well, once i got slightly bitten but that's another story lol, it was when she performed oral on me and she was a newbie) why not to relax, and enjoy some quality time everyone ? i mean, no risks, no pregnancies, no STD's if everything is clean... and they enjoy it, i enjoy it, then, why not ?
Well, that's what i think about this, i've been thinking about this recently, so that's the reason of the long post i guess, i'm having a struggle right now with my partner about this lifestyle, but hey, life's moving on.