Would you consider bestiality immoral then? The animal equivalent of bestiality (interspecies sex) occurs regularly between different species after all.
I am not able to provide an objective moral reason if other animals may be treated differently from humans. If consent cannot be taken into account, raping animals is not immoral.
The sole argument could be that bestiality harms or at the very least exposes an animal to a significant risk of harm. But then again, killing an animal certainly harms it much worse but this would be morally acceptable in such a system, so the harm an animal faces isn't really part of the equation.