Good, I can work with this.Sure:
Inanimate objects cannot express consent. Consequently, unless you are the owner of the object in question, anyone can lay claim to its affections. Who gets to decide who marries the Washington Monument, for example (Don't laugh, there are a lot of women who find it attractive, and size matters, no matter what women say to the contrary). If there is no means of deciding who has a claim to a public monument, then we end up either denying the right to marry the object or accept the premise that multiple people can marry it. The choice is then between no marriage to inanimate objects, or poligamy.
Even if property rights are not an issue (in the event that you wish to marry something that you already hold title to), does someone have to divorce an inanimate object before marrying again? Remember, no consent from the pet rock is possible, so it clearly cannot be an issue. That means that marriage, once entered into, can be unilaterally disolved. If that applies to marriages between people (and once the precedent is established, it eventually will), you end up with marriages that can be destroyed at will by only one partner. For an idea of what that looks like, take a look at Islamic divorces, where a man can simply publicly repeat that he is divorced three times and it's a done deal. The result of that will be families that collapse at the first disagreement. We'll end up with marriages that have no more meaning than casual hookups, with a complete collapse of familial responsibility, which, BTW, was one of the major arguments against no-fault divorce, one of the first steps on the slippery slope to where we are now. Precedents matter.
You have highlighted several good reasons why marriage between humans and objects is at best a non-sensical proposition. Property issues.. the fact that contracts are meaningless unless made between entities that are sentient and self-aware... heck, it might even raise strange issues in criminal cases.. would we charge someone for kidnapping for stealing an iPod that was someone's spouse?
We certainly can't stop people from having little ceremonies and declaring they have an intimate relationship with some object, but it certainly makes no sense to recognize it legally. One might say it makes about as much sense as creating a legal framework that allows one to loan money to themselves.
Now that we have a basic outline of the objections that pose seemingly insurmountable problems for proponents of object-marriage, we should take a look at the alleged slippery slope.
If the slippery slopers are to be believed, we should be able to plainly see how same-sex marriage will directly weaken these core objections to object-marriage that you and I have raised. If allowing X will remove our ability to object to Y, then the core objections to Y must be directly circumvented by X. In other, words, same-sex marriage needs to directly answer answer or provide a way around all (or at least the most important) of your core objections to object-marriage. It should be obvious to everyone, that it doesn't. But unless it does, our ability to oppose object-marriage has not been diminished.
Allowing same-sex marriage provides no possible answer to disputes that may arrise between two people who both want a monogamous marriage with the Washington Monument. Same-sex marriage provides no justification for a unilateral dissolution of all marriage, nor any other objection you have raised in regards to object marriage.
Object marriage is still just as incoherent and unworkable and will be just as easily opposed whether we allow same-sex marriage or not.
Hence, the fallacy of the slippery slope. This holds true for pedophilia marriage as well. Recognizing marriage between two of the same-sex in no way provides justification for disregarding all contractual laws and restrictions regarding minors, not to mention child rape laws. In other words, the slippery slope argument against same-sex marriage is simply BS... and there's no way around it.