It depends on how you define marriage. I can see why people could be opposed to same-sex marriage from an ideological standpoint, but 99% of the time it's just thoughtless bigotry. Marriage was traditionally intended to be an social, religious, and later legal institution designed to provide a stable framework for raising children. If you still believe in that reasoning then you could legitimately argue that same-sex marriage erodes that concept and is harmful.
Personally, I don't - I think the time when that was the primary for marriage is long gone, and now marriage is simply about people in romantic relationships, which is why I believe marriage is an outdated concept that doesn't properly reflect the myriad of different relationships on which people are dependent on each other and deserve legal protection. I don't think the rights and benefits provided by marriage should be limited to opposing-gender, heterosexual, unrelated, monogamous couples in a romantic relationship. I think relationships that may be same-sex, platonic, family-related or even multiple parties should all be entitled to those rights providing they meet a set criteria.
