Should it not be the states burden to prove that it wasn't?
The state's burden is reduced significantly since Mr Bryant has already, and in public, admitted sexual penetration occurred. The state has a witness (the accuser) who will claim it was not consensual. The state will meet a large part of the burden with that testimony. Add to it any physical evidence, such as bruising, tears, cuts, etc. The defense will have to find a way to rebut it or otherwise establish doubt.