With today's ruling that it is unconstitutional for Proposition 8 to preserve the timeless definition of marriage and instead create a “right” out of thin air, it is essential that we as Catholics can provide a coherent defense of of marriage in the face of the purely emotional “why can't I marry anyone I love?” argument put forward by the left.
If you are having a hard time with the basic logic of the argument which is quite possible since the left rarely argues in a logical manner, I suggest starting with a basic course on logic. This will be a great help in showing how a doesn't necessarily lead to c and a+a <> b.
Next, I suggest reading a primer on Theology of the Body. The whole issue of homosexuality being natural is based on a warped understanding of sexuality that has pervaded our culture. Theology of the Body for Beginners is one of the best starting resources for understanding that there is a distinction between sexes and it isn't a matter of choice. There is also a good book for Teens on this topic – Theology of His Body / Theology of Her Body that will help teens understand the unique nature of the two, and only two, sexes.
Once you have the foundation down, it's time to address the specific issue of same-sex “marriage.” Male and Female He Made Them is a Q and A book that succinctly goes through the whole issue. Marriage – The Rock on Which Family is Built is a more thorough discussion of the purpose and meaning of marriage that takes a hard look at the myth of same-sex marriages.
Basically, if rights are solely determined by preference then there isn't any such thing as an “inalienable” right, making all rights arbitrarily determined by judges who rule based on whether they believe something feels right instead of whether something is right.