If any two consenting adults marrying or being in relationship is always ok, then incest would be legalized. Mothers marrying sons, fathers marrying daughters, a brother marrying his sister. The argument "If they are adults, it's ok" is crazy.
In fact, most every argument for homosexual marriage can be used to justify incest.
What? Where does it say that the two people being married can be brothers or sisters? Last time I checked there were laws on the books forbidding incest. There is a huge difference between incest and gay marriage. Incestuous relationships are not only subject to religious taboo, they are demonstrably prone to offspring with birth defects, thus there is a harm to society.
I hardly think allowing gay marriage is going to create a huge demand for guys to marry their daughters and sisters (except in the red states which oppose gay marriage and where incest is the rule not the exception already).
