I disagree; I think saying God doesn't exist is much closer. Some people have a subjective belief God exists. Some of them admit it is subjective, others assert that God objectively exists. Others are saying no, it's not subjective, and, as a descriptive matter, we have no evidence God exists, etc.

Similarly, some people believe gender is subjective, and that people can "be" another gender if they feel like it (if I am getting this wrong lmk; it's obviously a simplistic statement of that position). Others believe the question is purely objective and determined by observable biology--i.e. "it's not subjective, and as a descriptive matter, you are x gender".

In contrast, "marriage is between a man and a woman" is basically purely normative. It is not a statement about what "is". It is a statement about what should be.