I can maybe shed some light. For starters, this is EXTREMELY common. Sexuality is a spectrum and as zoos we have more than one.
I am pretty extreme in both directions. That is, I genuinely do not find men attractive at all. I don't find the idea of sex with a man anywhere near erotic or a turn on. Being dominated, to be kissed or something by a man is honestly a bit gross sounding to me.
But with a dog, anything goes. Anything goes and it doesn't make me gay. I absolutely love anal play, receiving anal sex, having prostate touched while jerking off or being sucked off, etc. ... And while I'm usually the one wanting to dominate, I just make an exception for male dogs. A big, big exception. I turn into an absolute submissive bitch for a male dog, and I love every second. I want to be fucked in the ass as hard as possible by beautiful male dogs... The larger, and more muscular the dog the more I get turned on. The idea of dog cum, etc. is extremely erotic to me.
When I started really having sex with dogs, I wanted females mainly. I still would do so. But, as a man, receiving anal can be very pleasurable. I've fucked women in the ass, and know it feels really good for the dogs too. I slowly fell into it,... At first it was "better than nothing",... But the sexual experiences would invariably be so incredibly good...
I felt VERY STRANGE the first times I started jacking it to men getting fucked in the ass by dogs... But I had been having so much sex with male dogs and it always felt so good.
Finally, I started having sex with the same dogs enough that I developed deep connections with many of them. I realized that the more love between the dog and I, the better the sex. The better the sex, the more connected I'd feel. While it could be incredibly dirty and erotic, still - these dogs that I had deep connections with, I could suddenly have a foursome with other male dogs and it would be amazing, or a threesome with a female... That it was not only equal to human sexuality to me, but had a special aspect.... Since I don't and still don't desire men/human males,... Male dogs are the only ones that I will happily offer my asshole. No other animal or being does that for me!
Today I have had sex hundreds of times with male dogs, and with about 30+ different ones. I absolutely still LOVE sex with women too! ... I hope to combine the sexuality one day.
But again, it isn't uncommon at all. While I wouldn't mind sexual experiences with a man and dog,... As in, just helping another man become a bitch to a dog,... And I'm sure I could cum inside a man if I stuck it in... It doesn't really turn me on. Maybe sloppy seconds behind a dog, I could get off really good doing that maybe,... But still, I couldn't do anything submissive.