As for @TopKoak's comments, I think perhaps they're getting mixed up with an arc in Railgun T where Kuroko has her memories of Misaka wiped? During this arc she "claims" to be straight, but she pretty much immediately begins falling for Misaka again.
There's actually a really awesome scene later on in the arc where she basically embraces her feelings for Misaka and uses them to hive herself the resolve to beat an enemy