@TopKoak
If it helps, I can't find any evidence of that ever actually happening. Now granted, I haven't read the novels because Railgun is the only part of the series I actually like. Anyway, I think perhaps this idea has taken off because of two separate things
1: In Railgun T Kuroko's memories are manipulated and she's made to forget Misaka. This leads to her claiming she's straight but the arc ends with her falling back in love with Misaka
2: Apparently later on in the novels Kuroko kind of gets over Misaka? She still loves her but apparently she comes to accept that Misaka won't return those feelings. I can find no reference to her falling for anyone else, let alone a boy.