It is an organisation that advocates sex between men and underage boys.  This is not homosexuality.  It is paedophilia.  Is underage sex about sexuality?  One would surely take it more as about power and domination.  Do men of NAMBLA go home to happy and healthy homosexual relationships or are they married?  If the men of NAMBLA are all gay, then they are LGBT people and NAMBLA is an LGBT organisation 'by default' because of its membership, but then so is any organisation or group with a majority LGBT membership, regardless of the nature of that group.  NAMBLA doesn't qualify at all.