Unfortunately homosexual males are more probable to be infected with the HIV than heterosexuals and homosexual females; that´s a sad fact.
That said, homosexual males are a risk group, along with e.g. drug users or promiscuous heterosexuals. This assumption is based on sheer numbers and does not imply a statement of approval or disapproval of anyone´s lifestyle or sexual orientation.
As far as I know, the HIV cannot be detected for roughly three months after the infection. Plus people might consider themselves clean but aren´t, for whatever reason.
Taking all that into account, no groups of persons who, regarded as a whole group, are more likely than the average population to be infected with the HIV should be allowed to donate blood.
And please don´t lie when it comes to determining whether you´re a member of a risk group - the whole system relies on sincerity.