You are making a distinction that ignores the Church's idea that thoughts are forms of sin.  A homosexual orientation, as so famously described by Benedict XVI, is considered a form of intrinsic disorder.  How is one forgiven for being the intrinsically disordered person the Church condemns you for being?  Heterosexual adultery is still 'objectively ordered' and so forgiveness of sexual activity is easier to accept because it's easier to make the distinction between acts and self.  That was not an intellectual option for gay men of David's generation.  The sin was in who they were because they had the thoughts they couldn't stop.