> I could argue that anyone who supports any special legal recognition of "marriage" is directly pushing for the codification of discrimination.
There is a sense in which that is true -- every legal status is, by definition, discrimination in the law.
But then, there is nothing wrong with discrimination, per se, what is wrong is discrimination on bases which do not serve a legitimate public purpose. Which, of course, takes a lot more work to identify than mere discrimination, but then, meaningful distinctions are often harder than meaningless ones. And, so, we end up establishing a series of explicitly suspect bases for discrimination such that we require a higher standard of proof that there is legitimate need being served and no better way to serve it for certain forms of discrimination, because of our experience of them being deployed inappropriately to keep groups powerless rather than to advance some legitimate common interest.
> But why can't two roommates in a plutonic friendship receive the same benefits?
(1) the word you are looking for is probably "platonic", unless this is some kind of friendhip oriented around money, cartoon dogs, deities of the underworld, or dwarf planets.
(2) Generally, law doesn't require a romantic relationship for marriage, it just requires appropriate age (sometimes with exceptions or variations with parental consent), specific gender combinations (except where same-sex marriage is allowed), not already being in a marriage with a different partner, and not being within a certain degree of consanguinity. So, platonic roomates can get married.