Why does george bush think it is up to him to decide if 2 homosexuals can get married?:
Because Jr. doesn’t realize (despite having been told and having pledged twice) that his job is to protect, and defend the constitution of the United States.  He thinks his job is to do a variety of other things.  If you watch him speak for long enough, he will tell you what he thinks his job is, usually something along the lines of “to lead” or “to tell people what I am thinking” or “to unite Americans behind my common cause” or some other rubbish – never to protect and defend the constitution.

So to answer your question, Jr. thinks it is his job to decide if homosexuals can get married, because he truly honestly has no idea what his job really is.