I'm nothing but the other man
I am not sure what you mean by this.
I am drawn to girls who are already in relationships.
You should not try to get with a girl that is already in a relationship. You are going to waste your time, you should look for single girls to date.
I know they would be happier with me, but they do not know it yet.
That is something you do not know for sure. You are better off finding a single person to date and not someone already in a relationship.