It doesn't have to. Biology doesn't prevent any two people from loving each other, it may only prevent their ability to reproduce with a certain partner.
Not these f***ing stupid arguments again. A chicken is not a person, cannot consent to sex or marriage, and has no ability to apply any of the legal benefits of marriage, like property issues or power of attorney.
Who gives a s***? We're not going to make things illegal because you don't like them.