It is different, but it's not any less an injustice. One could even argue that it's worse in some sense.
Let me play the devil's advocate: an homosexual couple cannot conceive a child, so by giving then them the chance to adopt your somehow extending their natural rights. On the other hand restricting one's freedom of movement because one was born in the wrong country seems completely arbitrary.
Don't get me wrong, I fully support the struggle of homosexuals for equal rights, but why it's considered acceptable to maintain that non-citizens have less rights than citizens escapes me. It should be exposed as xenophobic behavior, exactly like campaigning against homosexual marriage is homophobic.