is an oft quoted defense of traditional marriage. However, it is the second half of the quote. It begins with "A man will leave his family and cling to his wife...."
This particular quotation can be gender neutral without loss of meaning. A woman can leave her family for her husband or a man for his husband.
This is the point of marriage and why arguments that families who have members living together can call the relationship a marriage ring hollow. The point of marriage is two-fold. First, it is the unification of two families before God in the creation of a new unit. I get her relatives, she gets mine. Any kids born or adopted are members of both. The other is the separation, as Jesus said, of both parties from their respective families.
When someone is married, their spouse has the right to use and disposal of all of their property, including their bodies upon death or their medical treatment if incapacitated. Families lose that right - as it should be. There is no need for family members to marry eachother to gaurantee that right, since it is already there. If and when medical decisionmaking is required for my mother, my siblings and I can do it without any additional ritual, unless my mother finds love at age 80, in which case her spouse has that right. My brother, on the other hand, had to go through tremendous amounts of paperwork (hopefully he actually did so) so that his partner rather than my mother has the say as to the distribution of his assets and medical care. What is missing is the ability to acknowledge this with a marriage through the joining of both families before God. Hopefully that day will come soon because it is good for the families.
Marriage is not so much about sex as familial politics. There is no reason for families with gays not to have their unifications recognized by their churches. (I so happens that both my brother and his husband are both Catholic, so their would be no mixed marriage problem). The church owes us recognition of their joining as much as it owes it to them, since they (hopefully) have done the legal footwork to shut us out in favor of each other.