This article has excellent points. It's rather long, but it helped me put my thoughts together on this controversial subject.
If we don't maintain and define "marriage" as between one man and one woman, preaching tolerance, and "acceptance of one's orientation," then, by default, we must allow anyone to marry anyone or anything. If we allow two men or two women to marry because "they love one another," then we must also allow a man who wants to marry two women, a bisexual who wants to marry a woman and a man, a man who wants to marry a young boy or girl (because I'm sure, in their own twisted way, they do sometimes love each other), a man who wants to marry his cousin, his sister, or even his daughter, or a woman who wants to marry her cat. Because if the qualification is "we love each other," if marriage is not defined as "between one man and one woman," then we must allow anyone to marry under the mantra of being "tolerant." Gays say we (conservative Christians) are intolerant, but they themselves are, because they would say that gays should be allowed to marry, but not any of the above who "love each other."
If we ignore 6,000 years of history (up until the last 20 years) of marriage being known as only between a man and a woman, it's a slippery slope of what is allowable.
And that is why I oppose gay marriage.
(Besides the fact that God's Word explicity says homosexuality is a sin. I Corinthians 6:9-11)