It all comes back down to morality.
Morality is a product of civilization. Our DNA hasn’t changed much in the past 30,000 years, but our concept of morality has. It’s been one incredibly prolonged discussion, with a few wars fought because the people commanding each army had different definitions of what is “morally right” and “morally wrong.”
So, when an animal enters a sexual relationship with another animal (of the same or of different species), they are not immoral, they are amoral. It’s not that they’re acting against a moral code, but that they have no concept (or, at least none that they can express to us) of a moral code.
On the other hand, a human being, part of a civilization that has been spending tens of thousands of years refining “right” and “wrong,” has a moral code. It may not be exactly the same moral code as that person’s next-door neighbour, but it can be compared and judged against other moral codes. In other words, we have reason to expect better of a human being than another life form. And a human shouldn’t engage in a sexual relationship without consent, even if the other party can’t properly conceptualize the idea of “consent.”
You bring up the point of dolphins, and I’ll throw apes into the mix. It’s entirely possible that they have language, and moral codes (I would argue the former is necessary for the latter), and perhaps even the idea of consent. If, at some point in the future, we can communicate with them, and gain that knowledge, then the moral question will change. However, for right now, the problem is that, even if they were able to conceptualize consent, there is no way to communicate it.
The most important part of the word consent is the prefix “con-,” meaning “together.” The act needs to be agreed to by both parties (or more, if that’s what you prefer). It’s a coin that can’t just have one side.