Ruphhausin, you bring up an interesting dichotomy, and the basis for the lack of a “National Law” for rape. From the legal end, the laws of the various states are quickly evolving over the definition of rape. Thank God we have gotten away from the old Common Law definition of rape: “Having carnal knowledge of a woman, by a man ,not her husband, against her will”.
Thus, under the Common Law definition, many jurisdictions actually required the victim provide proof that she had resisted to the utmost of her physical ability. (I say she, because, under the Common Law definition, men could not be raped; and there was also a nasty little side law known as “Spousal Immunity”, which has also been abolished).
Under that older system many rapes went unreported because of the requirement that the victim proving that she resisted physically. The old law also lead to some ghastly circumstances, which under modern statutory law, would be considered rape. (Think of forced intercourse with an unconscious person).
Thus, in an attempt to remedy this, the various state level legislatures began creating statutory definitions for rape. These definitions broadened the scope of what constituted rape, and took away the very strenuous requirements of proof of resistance. Needless to say, the development of the Rape Shield laws, and the broader definitions was something that was, and continues to be needed.
However, an unexpected consequence of that new found anonymity is the rise of false accusations. That being said, society does have a rape culture; and I do think that the old laws may have played some role in strengthening the foothold of that culture. The problem is, attempting to find a way to remedy the first wrong, without creating a new type of wrong; and there is no easy solution or answer for that.
With respect to a “National Law” for rape; it is true that there is no set federal sentencing for rape convictions. (Federal courts will usually apply the state sentencing guidelines where the crime occurred); however, there has been a recent push for a uniform definition of rape. That very well could be the first step towards a “National Law” for rape, in a sense. (see: http://sentencing.typepad.com/sentencing_law_and_policy/2012/01/will-new-federal-definition-of-rape-significantly-impact-approaches-to-sex-crimes-and-punishments.html )
It's such a difficult area of law, and I can honestly say it is one of the reasons why I refuse to do felony defense appointments. I honestly feel I could not zealously defend an accused rapist. (I still believe in the rule of law, and providing the best defense, but personally I couldn't do it). But the crime itself is an ever prevalent and concerning problem.