The difference between human homosexuality an inter species erotica is mutually defined consent. Humans are not entirely privy to the psyche of other animals, and we do not know what defines consent for them clearly enough to determine a legal basis for consensual sexual activity between humans and other species. The same goes for children and the mentally disabled, in my opinion. What may be one's species' consent is another species' lack thereof.
From the sound of this book, the dolphin gave as much consent as I would assume possible for an animal, but I don't know for sure, and I don't know if it happened the way he says it did.
Really, the issue in bestiality to me is potentially violating an unwilling animal, and causing sexual harm and trauma, not it being "sick" or "right" or "wrong". I think of sex with certain specific humans as more repulsive than sex with a dolphin, but it's mostly about the consent.
Homosexuality and inter species sex are both naturally occurring behaviors, just like heterosexuality and same species sex. The former, simply to a proportionally lesser extent.
Really, the most bashworthy thing is putting homosexuals and dog humpers in the same category. "If we allow gay marriage, we have to allow people to marry rocks if they want". Yes, slip on down that slope