[T]the state of the American family is deplorable and critical; if trends are not reversed, society will fall apart within very few generations. The largest problem is that many people have lost sight of the fact that marriage exists to join two families and start a third, and to ensure that--to the extent possible--every child has a mother and a father. Although homosexuals are not the primary cause of this decay, allowing homosexual marriage would make a bad situation worse.
Well, I don't think that most of it is terribly-eloquently written; I don't know how to best shorten it, though, without losing essential meaning. The last sentence is the key, though it doesn't make clear why it would make a bad situation worse.
IMHO, one of the key lies that advocates of homosexual marriage have used is that opposition to it is a result of a hateful belief that homosexuals are to blame for everything that's wrong with the world. Since homosexuals are not at fault for everything that's wrong with the world, that belief is clearly false and thus the opposition to homosexual marriage is predicated on a false premise.
The key, therefore, to winning over people to oppose homosexual marriage is to make clear that while homosexuals are not to blame for all the world's problems (or even for most of them), that does not mean that homosexual marriage would not be harmful to an already-badly-decayed American Family. Were the state of the American Family healthier, it would perhaps be able to withstand such harm without major effect. But in its current state, any additional harm could prove catastrophic.
BTW, I think another major point which needs to be made is that a legitimately-born person has one man to call "father" [living or deceased], and one woman to call "mother"; ideally, the person should be raised by these two people. Although sometimes Fate will intervene and preclude this ideal, and thus it may be necessary sometimes to make the most of a bad situation, that does not mean bad situations should be sought out.
If a child's parents were killed in a car wreck, and the only living relative was an uncle who happened to be gay, I would not necessarily be opposed to the uncle's raising the child. Not an ideal situation, to be sure, but being raised by kin is IMHO generally better than being raised by by strangers.
To my mind, though, there is a huge difference between saying that openly-homosexual people should be allowed to raise their kin in situations as described above, and saying that homosexuals should be given children to raise without regard to their sexual orientation and/or habits.