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It might be difficult to prove (in which case the alleged perpetrator would be found not guilty or it may never even go to trial).
But to DISALLOW EVEN PRESSING CHARGES AS A MATTER OF LAW seems unecessary and wrong.
For a little perspective, until the 1970s there was really no such thing as spousal rape as a matter of law, in part because of the sentiment that "once the woman has said yes, she can never say no."
http://www.vawnet.org/SexualViolence/PublicPolicy/SpousalRape.php