I said you shouldn't pass judgement. If someone was doing something harmful like watching porn, I do think you should help them to stop. While some of Katy's lyrics may not be appropiate, I wouldn't say they call for an intervention if someone you know is listening to her.
I listen to her because I've been a fan of hers since she was a Christian artist. I just like her stuff. While there are some things I don't agree with, it's not causing me to sin and it doesn't affect my walk with God. There are certain things that do, and others that I don't. Like I don't listen to Christina Aguilera anymore because she's way over the top with the sexual stuff. It was affecting me and causing me to think about things I shouldn't. Katy's song I Kissed A Girl doesn't make me want to kiss one. I think that song is actually quite juevnile. I don't like the other one you mentioned either, California Girls. She does have some very positive and inspiring songs as well. I do listen to secular music, but I pray everyday that God would give me discernment in my entertainment choices. I do often stop listening or watching things that are affecting me in negative ways.
Different things bother people differently. Using I Kissed A Girl as an example, that doesn't affect me for the reason I already mentioned. However, it probably wouldn't be wise for you to listen to it because you used to be bi and it might cause you to dwell on your past. I know some Christians who watch horror and vampire stuff and they have no issue with it. I, however, cannot watch that type of stuff because it affects me badly due to previous occult involvement.
When it all comes down to it, I just feel we shouldn't judge someone and assume something about someone. She has never, ever said she was bi. She has talked about the song, and said she was just curious. So until she says otherwise, we shouldn't assume. That's really been my main issue here is the judgement and assumptions. We can't just go around stating things like they are facts when we don't know for sure they are. That to me is not only gossip, but slander.