Interesting line of argument, and potentially quite powerful. If I may put words into your mouth, it seems that you are saying that marriage is an important social institution. By denying gays the possibility of participating in that institution, we are creating division within the community, which leads to instability of that community. Put another way, every person born into a community should be able to participate fully as a member of that community, if they choose to do so. By denying marriage to some people who are born into the community, we are effectively rejecting our own.
It seems a reasonable argument. Needs a bit of work, but has possibilities.
More to come.