How do I respond to the RFE with proof beside a copy of prohibited marriage laws of Virginia?

So, submit proof, if there is any, that it is legal for first cousins to marry in Virginia (it is probably not). If it is not, then you cannot enter into a legal marriage and, therefore, the fiance visa petition will not be approved. If there is another state where you are able to marry a first cousin and you have some ability to marry in that state, then show proof of that.

Can send the relevant VA statute that says your marriage is valid in Virginia, but I am not sure this is a legal issue you'd like to litigate on your own with DHS without the assistance of an experienced immigration attorney. For one thing, the fact that a marriage may be legal in one jurisdiction does not compel DHS' recognition of such marriage or else, Islamic polygamous marriages of Saudi Arabia should be accorded legal recognition by DHS.

Show them that marriages between first cousins is valid in Virginia by providing them a copy of the law where it states that it is legal. In the alternative, you can contact a registrar's office in Virginia and ask them to provide you with a letter from the jurisdiction, on their letterhead stating that it he marriage between cousins is legal.

Yes, you can submit a copy of the statute (Va. Code Ann. 20-38.1 (2010)). The statute outlines which marriages are prohibited. Any marriages not listed by the statute, by implication, are permitted.