The notion that homosexual marriage would somehow in someway affect heterosexual marriage is wacky nonsense.
Actually, I think it does affect heterosexual marriage: legalizing marriage for everyone sends a strong message that marriage is an important right that shouldn't be tampered with. Making same-sex marriage illegal sends the message that it's good and proper for the government to be ruling on who should be married and who shouldn't, and that our rights are negotiable, subject to the whims of people's aesthetic preferences.
The implicit message in banning same-sex marriage is that governments can deny people the right to marriage based on nothing more than people's personal preferences and their opinions about the people in the relationship in question. This is a threat to the institution of marriage as a whole: if marriage can be denied to same-sex couples for no good reason, then it can be denied to anyone.
With same-sex marriage legalized, the right to marriage is protected for all. With it banned, the only real protection I have for
my marriage is the fact that my society doesn't generally have a problem with marriages like mine (opposite-sex couple, first marriage for both of us), however, I'm not comfortable with the idea of trusting that something as fickle as societal tastes will
never change in a way that disadvantages me at some point in the future. I want my marriage protected by something more than this, but the prohibition of same-sex marriage sends a clear (IMO) message to everyone that if enough people disapprove of my relationship, then they can strip it of legal status and protection.
For selfish reasons
as a heterosexual person, I find this unacceptable. I want my marriage protected regardless of whether people like it or not.
Edit: if we build our laws using the principle that a person's rights can be denied simply if he or she is disliked enough, then none of us really have any rights at all.