i do not agree that the government should not discriminate in deciding who should acknowledge as married under the law. i am pretty sure you also do not agree that the government should not discriminate. for example, should the government prohibit a father from marrying his daughter? the government currently discriminates against a father and daughter who want to marry. are you saying it should not discriminate in that instance?
it seems obvious that the government can rightly discriminate in deciding whose relationships it will sanction as civil marriage.
so saying the government should not discriminate is not a valid objection.