Philosophical definitions can be flawed, and there is still ambiguity. Furthermore, the definition I gave before was not necessarily philosophical, so leaping off of it as if it were is problematic.
The definition you gave before wasn’t a definition of justice but of discrimination. And it wasn’t a “legal” definition, either, you just copied it out of a (non-legal) dictionary. So I hardly see complaining how my understanding of justice isn’t a “legal” one in any way invalidates my pointing out that your (equally not a legal definition of) discrimination doesn’t preclude the claim that heterosexual couples are discriminated against by virtue of gay “marriage” laws.
To wit, here’s the definition you provided: “discrimination: the unjust or prejudicial treatment of different categories of people or things.”
OK, so if discrimination amounts to the unjust treatment of different categories of people, and if homosexual “marriage” is in fact unjust, then the arrangement is discriminatory, especially when you consider what I wrote in the same post:
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This is especially true when you consider that the things that normally accrue to married couples with families, such as tax benefits, are part of a kind of zero sum pot. If government spending remains constant, expanding the pool of people entitled to tax breaks simply for being married must necessarily either (a) entail a cut in government spending and thus benefits, (b) entail a cut in the size of the tax break, or (c) entail an increase in debt which must eventually be retired by means (a) or (b).*
In other words, we don’t have an infinite supply of stuff we can give out for free. We have a limited amount of stuff. Increasing the number of people given stuff decreases the amount of stuff each individual person gets. Given that one group of people (heterosexual couples) have a legitimate claim to that stuff and another group of people (homosexual couples) don’t, by decreasing the amount of stuff given the former to give stuff to the latter you have (a) unjust (b) differential treatment of two groups of people. Which is discrimination, by the definition you provided.
Whether the definition you provided is objectively correct is not something I care about. In fact, I don’t care about “discrimination” at all. Where discrimination is not unjust, it’s not bad. So our main concern should not be whether or not gay “marriage” is discriminatory but whether or not it’s just. It isn’t just, therefore it’s bad regardless of its discriminatory character.
That does not mean heterosexuals are discriminated against. That simply means that homosexual marriages are given the same status as heterosexual marriages.
Again, I don’t care whether this arrangement constitutes discrimination. That’s pedantic trivia to me. I care about the fact that it’s an injustice.