Again, you making extreme illogical jumps here. How do you go from banning gay marriage to waging war with muslims?
We should ban gay marriage (or better, prevent its passage in the first place) because we believe that to be the way a just society functions. Our societies are not free-for-alls. We restrict all kinds of activities deemed harmful: theft, murder, arson, speeding, fraud, etc. Catholics consider homosexual marriage to be within those same categories because they are just as harmful. Homosexual marriages add nothing to society because they do not contribute back to society in any meaningful way. Heterosexual unions under normal circumstance do by producing children. Hence, we encourage and protect those unions because they mean the furtherance of our society. This is a big part of the natural law argument against homosexuality.