Opposition to monotheism has many reasons and forms. It is reckless to group them together, and odds are that it will lead you into mistaken conclusions.
The exact same thing is true of Christianity, Islam, or any other religion.
I don't think it will serve you well to take as a premise that "religion" works as a general descriptor, personally. There are very worthwhile doctrines out there, and there is ritualized, destructive insanity, even entering clearly criminal territory. It serves no one particularly well to treat them all as proper examples of an abstract "religion" category.
All of that applies equally to Christianity, Islam, or any other religion. What beneficial purpose does it serve, for example, to treat everything that anyone calls “Christianity” or “Islam” as proper examples of an abstract “religion” category?
I would like to find a better name though, for the religion I’m thinking of. I considered “Materialism” because that’s what some of its leaders and followers have called it themselves, but I’m not sure that would be a better name for it than Antimonotheism. Maybe if you ever see what I’m getting at, you can help me find a better name for it. In order to be what people call a “religion,” it would have to encompass everything that anyone says and does under banners of “science,” “reason,” “skepticism,” “free thinking,” “evidence,” “research” or “logic,” for example, whether or not they actually practice them or even believe in them.
You may be mixing uninformed, deluded scientism (which is quite common) with misunderstood or poorly understood science proper (which is also very common, but quite a different beast).
If you think of people as “Christians” or “Muslims,” you may be mixing uninformed, deluded attitudes and behavior under those banners (which is quite common) with misunderstood or poorly understood religion proper (which is also very common, but quite a different beast).
ETA:
“Scientism” might have been a good name for it, if that name had not already been applied to a subset of it.