I've wondered about a few people over the years. I had a housemate who owned an intact male dog that slept with him in his bedroom, which was next to mine. One night I heard him making sounds of sexual pleasure, and they weren't the kind of sounds you make when masturbating, but the kinds of sounds you make when someone else is giving you pleasure, the kind of sudden gasps and sighs you make when your partner touches you just right, or does something unexpected that gives you a jolt, or when the sensations become a little too intense. But I knew he was alone in the room except for his dog. Nothing like that ever happened again, though. My best guess is that maybe he was just experimenting that night, if anything really happened at all. He did mention to me once that his dog had a "drippy dick," but I don't remember the context of that statement. Otherwise, as far as I knew, he was a "normal" straight guy. He moved out to live with his girlfriend who lived on a farm. Eventually they got married and had kids. At one point they were running a pig farm together, but I lost touch with them years ago, so I don't know if they are still doing that.
This housemate's best friend had a female dog and I just sensed that there was something going on between them. He wanted to mate his dog to my housemate's dog, but my housemate seemed to be on the fence about it. Furthermore, for some reason the male dog showed little interest, although the generally got along and seemed to enjoy playing together. One evening she seemed to get aggressively flirty with him and he ran into my bedroom and hid behind me. I don't think she was in heat, but maybe she was approaching heat? As far as I know they were never brought together when she was in heat. She got kinda flirty with me, too, sometimes. I remember one time when we were alone together--the other guys were outside--she rolled over on her back as if presenting her sex to me. At least that's what it seemed like at the time, like she was giving me an invitation and waiting for me to accept. I rubbed her belly and tried touching her vulva, but she seemed to get nervous and kept looking at the front door (her human was just outside), as if she was worried about being caught "cheating" on him. (Or maybe she was hoping he'd come in and rescue her from my unwanted attentions.) Anyway, since she seemed a bit uncomfortable I backed off. Besides, we were in the middle of the hallway near the front door and they guys were just outside and could have come in at any moment.
Anyway, one of my other housemates told me that this guy casually told her that he wished he could marry his dog--apparently he'd said something like "I'd marry [dog's name], if I could." She thought it was weird, but she didn't interpret it as indicating that he was having a sexual relationship with his dog, just that he was content with his dog as his primary companion. I don't know the fully context of the conversation, but I understand it he was saying that he didn't need a human wife--his dog was enough for him--although he did sometimes express interest in finding a girlfriend. But he also felt that if he was able and willing to "settle" for a dog as his primary companion, he should be able to marry her and make it official.
Many years ago I briefly dated a guy who had a couple of dogs. He had an on-again, off-again boyfriend. (This was during one of their "off" periods.) The boyfriend was apparently kind of weird and had a bit of a drug problem, and the relationship seemed very dysfunctional. I asked why they kept getting back together and he said something about liking guys who were weird and how they knew things about each other they couldn't share with anyone else and "he's the only guy who really get me" and so on. It didn't occur to me at the time, but later I began to wonder if the thing that they had in common was a special "interest" in dogs. This is partially because I know they were friends with an older gay man who lives up the street from me. This guy lives alone with a bunch of dogs. I've never actually met the guy, but I see him several times a day, walking up and down the street with his dogs, and if I'm out walking or driving by, we wave to each other in passing. I can't help but to wonder if he has sex with his dogs, but maybe they are just pets, and I just think they're having sex because that's what I'd be doing with them if they were my dogs.
That's all I can think of at the moment.