Since the sash is a symbolic way of claiming publicly that you are not in communion with the church this makes perfect sense to me.
From Yahoo! News - Gay-Rights Activists Denied Communion:
"Priests at Holy Name Cathedral in Chicago refused to give the Eucharist to about 10 people wearing the sashes at Sunday Mass. One priest shook each person's hand; another made the sign of the cross on their foreheads.
'The priest told me you cannot receive communion if you're wearing a sash, as per the Cardinal's direction,' said James Luxton, a Chicago member of the Rainbow Sash Movement, an organization of Catholic gay-rights supporters with chapters around the country.
An internal memo from Chicago Cardinal Francis George that became public last week instructed priests not to give communion to people wearing the sashes, which the group's members wear every year for Pentecost. The memo says the sashes are a symbol of opposition to the church's doctrine on homosexuality and exploit the communion ritual. "