Gender does not need to be part of the standard. And it would be a poor choice to make it so. Gay couples buy
DVC. Sisters buy DVC. There is nothing that says the the couple needs to be a straight married couple - or a couple at all.
They should simply list members according to who signed first on the paperwork, mark that as the primary contact. I've never gotten mail personalized by DVC where they've had to use a pronoun to refer to me (or my husband). The most they need to do is put a title there - and with the many choices in title (Mr., Miss, Mrs., Ms., Dr., Reverend), the capacity to personalize a title is there. I'm not sure what the Bible has to do with anything, other than providing a rationale for how we ended up with such a tradition.
And it may not matter to you, but for someone whose marriage is not stable, who bought with a friend where the friendship doesn't last the term of the contract, have a son that they bought with, but thought they'd be the primary contact, receiving the mail may make a big difference.