Also, I do not see any kind of an index that has a fifty-fifty split on cardinality being helpful, in the case of M, F, on any database product. I have created indexes on flag fields (Y/N), but in those cases 95+% of the table was one response or the other and we were looking for the value that was on less than 5% of the rows. For instance, Y(es) the row has been processed and we are only looking for the rows that have N(ot) been processed.
Dave