My wife and daughter are zoo-exclusive.
What happened was my wife and I hooked up 20 years back at a party and living in a rural area you can guess what happened. Our parents dragooned us into getting married and we were never very happy together. Sex between us pretty much instantly dried up and I cheated on her pretty openly, she said she didn't care anyway.
After a while my wife found zoo porn on the internet and after some time decided to give it a try. We had a 2 year old English Mastiff named Samson at the time and she got him to lick her first and finally mount her. I noticed a change in her demeanor immediately, though I didn't know why until some time later, she was happy and much more fun to be around. I became much more attracted to her, but we only had sex once after that, my wife didn't orgasm and always turned me down after that. I figured she was having an affair, but I didn't have any right to object after all the cheating I had done so I left her alone. I eventually caught her having sex with Samson, thought it was so hot, and after I calmed her down assured her that I was happy for her to have sex with the dog if that's what made her happy. We moved into separate rooms and she has never looked at a man since.
My daughter is a bit more extreme, once she got much older my wife eventually let slip her activities with several dogs by that point and my daughter became interested in the zoo-lifestyle as well. She has her own dog, a Great Dane she calls Rocky, and he apparently took her virginity. Then after a year she started doing this thing where she would act like a dog, she had one of those butt-plug tails and paw mittens. She would apparently do this all day while her mother and I were at work, my daughter had failed her last year of high school and hadn't gone to college. After a few months of this she told us she wanted to do it full time, live as a dog. She didn't think she had any better prospects in life, I loved her to bits but I think she was right. My daughter isn't smart or attractive and she's very socially awkward and generally unhappy. Since she started this living as a dog lifestyle she, like when her mother became zoo-exclusive, is much happier.
So ya that's how the two women in my life became zoo exclusive and they are both much happier for it.