If they didn't believe it, why'd they give her that title?
Does the Church typically confer honors and titles on people which they know are false because they don't *really* believe it?
Because she had a Child from Him. But it is not a full-fledged belief, because we don't believe everything nornmally entailed with being a spouse: she was and remained married to Joseph, there was no trappings normally associated with marriage, -- no shared household, no mutually shared duties, etc. In a certain limited sense, one can say that she was a spouse of the Holy Ghost. In many other senses she wasn't. It is a unique situation.