This stems from the misconception that marriage is between two people. It is not. It is between two people and everyone else, who must accomodate, recognize, endorse, facilitate, and institutionalize for them.
Why does this "hurt" them? I really don't understand their prejudice. No one loves a person like their dog. Unconditional love that, is not found in humanity. Dogs don't argue, dogs don't demand their own way. If someone finds pure joy and happiness with their dog and wants to share their life with them, why should this upset this homosexual couple. Why should they be "hurt" by the private relationships of another person?
Could it be that legally equating the interspecies love parterners, somehow degrades and makes a mockery of the "solemn" nature of her relationship? Because in fact interspecies relationships are NOT normal, and are in fact aberrant..deviant?
Well now. If she can understand that, then she should be able to understand that this is exactly how Hetrosexual couples feel about their marriages. It "hurts" them to hear homosexual partnerships described in the same terms as theirs.