There should be no reason why a male goat, horse, or other barnyard friend cannot marry a woman. If the barnyard friend makes the first move on the woman and she consents, you have the consent that is necessary. Once the rules have changed from one man and one woman, anything should go.
It is problematical with a female barnyard friend. How can a male human prove that the animal gave consent?
I don't know how to evaluate a homesexual male human getting together with a homosexual barnyard friend. And transgendered barnyard friends really loses me.