Okay, here's how... Once we redefine marriage to include a sexual behavior, then any sexual behavior can be legitimized through a marriage. This means, people who feel they have a right to marry their horse, should be able to do so, it's their sexual preference. We have defined marriage on the basis of sexual preference, so the standard has been set already, and it's just a matter of distinction. The Constitution prohibits us from passing laws which only apply to certain people, and not others like them. If it becomes constitutional to allow marriage on the basis of sexual preference, then that's the law forevermore. It doesn't matter if the sexual preference is something you aren't comfortable with, a right is a right.
Secondly, when our society adopts "gay marriage" as an alternative, future generations may choose to take this course, as opposed to finding a traditional marriage partner. Over time, it may very well become the "norm" for young people to marry the same sex, regardless of whether they are homosexual, just because it is easier and less difficult to make such a union work. No children are involved, so you wouldn't have custody disputes if things didn't work out. Decades roll by and the trend continues, to the point where traditional marriage is almost unheard of anymore, people just choose not to go that route... voila, no more family structure in society. Without the family structure in society, fewer children are born, and those who are, will be taught it's better to be homosexual or marry same sex, to avoid the traditional problems of marriage and live a happy life... a few more generations pass, and we have no society anymore, we become a dying civilization.
Now you can claim that I am being overly dramatic and this stuff would never happen, but there is the problem... you aren't willing to face the possible ramifications of your stupidity because you don't want to. It's easier for you to imagine there would be no repercussion, and life would go on as usual, but no significant change such as this has ever happened, without causing a ramification or consequence, it just isn't realistic to imagine it could.