“If someone values pets more than people, how is that idolatry?”
You are valuing a human created in God's image below that of an animal not created in his image. Christ made it clear in the gospel of Mathew – among many other places – humans are worth more than animals. This doesn't mean we can't love them, care for them, and derive enjoyment from them, but that we should not value them above humans.
“You say humans are made in the image of god (a claim you can only support by appealing to your specific holy book),”
It's not my " holy book " but God's word. and that appeal - although your words indicate doesn't amount to much - is worth more than all the literature in all the higher learning institutions in the world.
“but that's different than being on the same level of value as god, right?”
All people are created in God's image or likeness. The image of God (Latin: imago dei) refers to the immaterial part of humanity. It sets human beings apart from the animal world, fits them for the dominion God intended them to have over the earth (Genesis 1:28), and enables them to commune with their Maker. It is a likeness mentally, morally, and socially.
Mentally, humanity was created as a rational, volitional agent. In other words, human beings can reason and choose. This is a reflection of God’s intellect and freedom.
Humans are the most glaring representation of God. When you value an animal above this representation - you're committing idolatry. Which is making God into your image vs him making you into his.
“As far as something being a subjective preference, yes, there are things that are sometimes subjective.”
Not when it comes to the value or worth of humans above animals. However, if you reject God's word - it is in fact reduced to a “ subjective preference “ and that carried to it's logical conclusion is disastrous.
“So what. I think lima beans are gross. That's a subjective statement.”
You're comparing the arbitrary preference for a food to the objective value of a human being. This is the conflation of separate issues.. However, human worth becomes just as arbitrary and of the same worth, apart from God.
“I'm not saying they're objectively gross to everyone. But to me they are.”
That's good to know, but the problem is, food preferences, tastes, and human worth are entirely separate issue, but again, if you jettison God, they're both just as arbitrary a preference – one worth no more than the other.