Neither in the USA nor in Canada have opponents of same sex marriage been able to demonstrate a valid reason for discriminating against same sex couples with respect to marriage. Because they can’t have children? That doesn’t work, because heterosexual couples can marry even if they are too old to have children, or won’t be having children for some other reason. Because social harm will result from permitting same sex couples to marry? No one has been able to demonstrate any social harm. For example, permitting same sex couples to marry will not block heterosexual couples from marrying. Nor is it likely to increase the divorce rate, which is already ridiculously high among heterosexual couples.

But if a father marries his daughter, inbreeding leads to birth defects and degrades the gene pool. There are also potential problems of consent, depending on the age of the daughter and/or how much she is controlled by her father. Those are valid reasons to discriminate against fathers who wish to marry their daughters.

How about a man marrying a sheep? Now we’re talking about a completely different scenario: a human being marrying some other kind of animal. That’s not the same as a man marrying a woman, or a man marrying another man, or a woman marrying another woman. As they say on Sesame Street, one of these things is not like the other. We can establish different rules (marriage is permitted in one case; but not permitted in the other) with respect to two entirely different things (marriage of any two human beings; marriage of a human being and another species of animal).

So your arguments are “straw men”. You’re trying to justify a pre-existing prejudice by introducing false equivalencies, or making an ad absurdum argument.