Why does a relationship with a non-human differ from a relationship with a human? Because one is human the other is not, because one is self-aware/ highly intelligent and the other not, because A is superior and B is inferior? I have always found humanities focus on itself egotistical and highly superficial, we have a literal history of discrimination, slavery, judgement, and so on to each other and yet the moment any sort of feelings towards non-humans is brought up many argue from a point of view that assumes humanity is harmonious when it clearly isn't.
Now to answer your question, why does it matter what species a creature is in any form of relationship (this is not me condoning bestiality btw that is gross and wrong on many levels). What makes a relationship with a human so much superior to non-human because from my point of view it really isn't and the very idea of human superiority is beyond stupid as it is example narrowminded egotism seen throughout history to endorse discrimination such as the enslavement of African Americans. This vary notion of relationships with non-humans being of lesser value is something ingrained in our culture maybe even ingrained on a genetic / instinctual level in order to ensure the progeny of the human species.
I never understood peoples attachment to their own species for my dogs / cats to me are honestly family as important as my mother, father, or brother. I don't feel the need to constrict myself to my own species for whatever reason whether it be because of my surrounding cultural influences, or a deep seeded instinctual desire to procreate. I view all life as having it's own way of being, its own value, and that it isn't superior or inferior to humanities way rather it is just different. I don't view my relationship with my pet family to be one of pros and cons as that is just a sure fire way to create examples of discrimination due to this overly contrived notion of "Why would you allow yourself to attach to a non-human? You find being attached to a non-human reliable?" Why does it matter if they are reliable or not, why shouldn't I attach myself to non-humans, what makes humans actually superior, we all live and then die that is an absolute truth, we breath, eat, drink, play, and so on our self-awareness though certainly unique on earth should not mean that can trample on all life as some superior life form or else how is it any different from the Romans belittling the Barbarians. If reliability is the standard on which relationships can be judged then how is a relationship with an autistic child, a crippled war veteran, and so on 'reliable' my brother is autistic and will never be able to work, be in a intimate relationship with another person so does that mean I shouldn't attach myself to him due to him being unreliable? Of course not!