Because government employees represent the government while at work. The Scouts don't believe the open practice of homosexuality is morally acceptable, meaning that an openly homosexual Scout leader cannot truly represent the Scouts. Probably not any more than most people believe the Scouts promote arson by teaching boys how to build fires. Groups like United Way need to remember they were not founded to police anyone - we have courts and legislatures to do that. Ultimately, public opinion will decide.
People can donate to any group they choose - even bypassing any clearinghouses if they choose.