It is effectively a ban. You can not advance through the ranks as a boy scout if you can not hold leadership positions.

As an example, here are the requirements for eagle scout. Check requirement #4. http://www.usscouts.org/advance/boyscout/bsrank7.asp

Compare to the girl scouts who have held this position on sexual orientation since 1991:

As a private organization, Girl Scouts of the U.S.A. respects the values and beliefs of each of its members and does not intrude into personal matters. Therefore, there are no membership policies on sexual preference.