Please, don't think I am defending him, but he isn't arguing from the point of harm...I think he's arguing from equality.
This is a word you guys freely throw around, so the natural question would be: "Do you think we're equal if you can marry who you want, but I cannot marry who I want?".
True, I find incest quite ridiculous and harmful, and animal marriage as simply stupid, but this is an issue in the name of equality, not harm.
The thing that you seem to be glossing over is that we're speaking about marriage in the terms of a legally binding contract recognized by the government, not just equality in terms of the relationship being recognized. Animals, objects and children cannot legally enter into a contract, so right off the bat we can eliminate them from the conversation. You may think a relationship with an animal or toaster deserves the same recognition, but in the eyes of the law, those things can't enter into legal contracts, so you could never actually "marry" them in the eyes of the government. If you're talking about marriage from a purely ceremonial standpoint, marry all the toasters you want, just don't expect the court to grant your estate to an appliance in the event you die without a will.
Incest is a sticky wicket. On the one hand, I don't honestly care if two people who are closely related have a sexual relationship. But most places in the country have made it a crime for people to reproduce if they're within a certain degree of relation from each other, and approving marriage for those people could be seen as a tacit endorsement of their right to reproduce; even though reproduction isn't required for marriage, the contract of marriage does include stipulations for offspring produced from the assumed sexual relationship of the people entered in that contract. If you're talking about the legal recognition for a sexual relationship where reproduction between the partners is considered a crime, you've entered into a very strange place indeed; OK, you're allowed to have sex, but if you accidentally get pregnant, it's off to jail! That's an odd scenario, no? I don't really have a solution, but I think it's safe to say that it's an infinitesimal percentage of the population that has a vested interest in this, so we can probably skip it for now and let the academics figure out the logistics.