The states specifically granted powers long ago to tax incomes and raise armies to the federal government.
They granted no power to define marriage.
No, but they granted the federal government the power to enforce the Equal Protection Clause. That clause was used to strike down the marriage laws of numerous states in Loving v. Virginia (1967) (striking down anti-miscegenation statutes).
I am not saying that Prop. 8 is unconstitutional. But your original argument-- that federal courts have no power to review state constitutions-- is simply wrong.