Allow me a few thoughts if you will. I realize this is coming from someone with no real-world experience but hear me out. First, the psychological aspect of being mounted, owned or submissive to a k9 does have a naughty/taboo factor. That along is enough to create a highly arousing, erotic experience that for many women could be sufficient to be worth the effort. Second, every woman is different, but I would imagine the knot feels pretty amazing grinding on the g-spot. Even when the dog isn't thrusting, just the twitching and pulsing could be enough to produce an orgasm for some women. And even if not, I know that a lot of women like to relax, dwell in that head space I mentioned earlier, embrace the naughty and rub their clit to get their orgasm (or more orgasms). Third, role play - since the brain is our biggest sexual organ, simply putting yourself in a submissive "bitch" role with a male k9 can be fun. I'm sure this aspect alone could be enough for some women to get them hooked on it.