The Name of Christ is indeed offensive because of what it means -
you have no hope of attaining righteousness on your own.
Nothing you can do on your own will save you from judgment.
Well, that is not their reasoning. By their reasoning, to say that God has a Son is equivalent to polytheism, so they think it is blasphemy.
If they were right, it certainly would be worse than fornication or drinking alcohol. They just aren’t right.