I'm going for a guy who has a thing with another girl
Why would you want to get involved if he is already in a relationship?
I want him and no other girl is going to stop me.
If this guy you want has another relationship already, then it's wrong to try to steal him away from his gf. Look for your relationships with people who are single.
I'm going for him regardless of the current relationship.
I think that the safe thing to do would be to back off. It is not okay to even try to steal someone's boyfriend.