The term "known homosexual" makes it sound like it's a crime to be gay, or that it's been "found out" and is something to be hidden.
Not to mention all the unknown (to you) homosexuals that are in contact with children all the time without incident: gym teachers, swim coaches, librarians, cafeteria workers... it's blatantly discriminatory and, while they have the choice to enforce such discrimination in their club, people also have the choice to sign their kids up to an organization that will expel any member alleged to be gay, and I certainly have the choice of supporting that group in any way.
What does that kind of discrimination do to the 12 year old who's started having homosexual feelings? To know that if he talked about it and showed honesty in doing so (that's a badge, right?) that he would be kicked out of the club doesn't send a tolerant or positive message to kids. The scouts need to come into the 21st century already.