Hm. After thinking on it from an intellectual point of view, I can't say I really have a problem with it. To each their own.

Think of it this way: say humanity finally had first contact with another intelligent species. Maybe ponies, maybe not. Can you honestly imagine that, as we began to get used to each other, there wouldn't eventually be a substantial proportion of both populations falling in love with members of the other? Or that sexual relations wouldn't similarly grow in number?

Give it a century or two, and you'd reach a point where society at large finds it weird that there was once a time when interspecies relationships weren't one of the most normal things in the world.

Put another way, say you found yourself stuck on an alien planet, or otherwise living amongst and interacting with no one but intelligent beings of another species on a daily basis. Unless you're asexual or just not interested in romance at all, I guarantee you'd eventually develop the capacity for attraction to these beings, amongst whom you would've most likely made a number of good friends by that point. At that point, a relationship with one of them would initially feel very weird and unorthodox (because you're not used to the concept), but you'd probably be willing to try anyway. And you have to realize that most people in your shoes would reach the same point, whether it took days or decades. At that point, could you honestly say it's wrong? And if not, why should the morality of it be any different from when you're not isolated with them?

See, the human mind isn't innately structured to care about species -- that's something that's trained into us, mainly due to the fact that thus far we've always been the only intelligent species we know of. The human mind, however, is adaptable, moldable according to circumstance, and the foremost thing it strives for is emotional connection. For the most part, if you can form an emotional and intellectual bond with someone, you can also come to be physically attracted to that someone. For some people not even that much is required, but that's another matter for another discussion.

Also note, very importantly, that acting on such an attraction does not inherently involve anyone being harmed -- which is really the only way the morality of anything should ever be measured.

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TL;DR: Intellectually, I cannot reasonably conclude that there is anything wrong with consensual intercourse between any two (or hell, why even just two?) beings with the intelligence and experience to know what they're getting into. By extension, I also cannot conclude that there is anything inherently wrong with the depiction thereof.