Such a contract has no good reason to be limited to just describing traditional marriage or even having that much to do with sex or even raising children

Incompatible values. Some people hold values that say e.g. having sex outside marriage is evil in and of itself. And of course to qualify, "marriage" has to have certain properties that they will recognize. Similarly, some people do not accept (in a values sense) the separation of church and state.

Some people who object to widening the scope of what is legally considered "marriage" do so because only some kinds of marriage have properties that are deontologically important in their morals (as a judgement of others, not just as actions to be taken themselves).

In my experience, there's not much use arguing directly about differing deontological moral values.