Well, Ginny could be an outcast, and rejected by her family for something (ie voicing an opinion on a matter, or just plain left out - not having friends...) and so she goes a little evil, and doesn't wanna be like her family, and maybe Lucious kills someone close to Draco, so he turns on the dark side (not leaving completely, but just not as evil) which makes them more like eachother.

Plus, who said that Ginny doesn't like "bad boys"?

IT COULD HAPPEN!!