It isn't hard to grasp. It's trite, in fact. The problem is that you seem to want to use it as an excuse for defending Kerry's proposal of second-class marriages for gays. Gays should be able to get married like everybody else. Religions can decide for themselves if they want to adopt this policy, too. The point is that marriage is now a publicly-sanctioned institution, and all adults should be able to acquire it. Quite simple. Why are you having so much trouble on this?