The point is that the institution of the marriage relationship, as it has arisen from the physical nature of human beings, is between one man and one woman who together share the responsibility of having and nurturing children, again as they were naturally designed to do.Homosexual relationships are contrary to that; they are contrary to natural design and as such stand apart from that unique relationship that is the basis of the concept of marriage.
Does that still carry your point even though it was refashioned in secular terms?
While I personally am unable to remove God from any part of the picture, I appreciate your trying to make my post more understandable to folks who might be coming from different belief systems. :-)
My main concern is that people might let the question "But what about married heterosexuals who do not have children?" distract them from Keyes' point.
Hopefully both our posts will help prevent that from happening!