Eurgh I'm not sure if I have explained this in the right way.
Put simply, the reason why heterosexual unions are treated as special, is that they do have a benefit on society, a positive(rather, essential) one at that.
Gay unions, like your and my friendships offer society nothing, and as such shouldn't at all be eligible for equal treatment, owing to the basic fact that they are not equal. Not only to gay couples earn more on average than heterosexual couples, the vast majority do not have kids and are never planning on adopting them. Why should they merit the same benefits which are given to heterosexual unions then?
(And please don't bring up the whole gays are people too thing, because yes we know a gay person is equal to a straight person, but we're not talking about people, rather unions between two straight people vs two gay people).
Now just because they offer soceity nothing, doesn't mean gay unions can't be allowed, its just they should always be subordinate to heterosexual marriage for the simple fact that gay marriage only benefits gays.