I'm not in a heterosexual marriage right now, but I was in one for eight years, and I'll be in another one starting next year, so I think that counts.
And the notion that gay marriage "threatens" heterosexual marriage is, quite frankly, stupid and pathetic, and anyone who believes that is full of horse manure of the smelliest order. This is bigotry, pure and simple. Anyone who objects on religious grounds is also full of that same horse manure, unless they also don't mix wools and linens, believe in the death penalty for adulterers, and keep kosher. Oh and, if they're men, always wear hats. If not, they're cherry-picking the parts of the Book of Leviticus that suit their pre-existing prejudices. That, folks, is bigotry, and to call it anything else is to let the bigotry win.