[dba-SQLServer] Indexing on a small number of values in a column

I have heard or read that creating an index on a column containing a small number of values is pointless.

a) Given a column Sex with a constraint "M/F", is there any point indexing this column?
b) Given the same column defined as a bit, you can't index it (afaik).
c) Assuming that it is pointless to index a column containng < n distinct values, what is the value of n? Assume 100M rows in the table of interest?

TIA,
Arthur