That's the 'how', not the 'why'. I think biological incentive here means, the reason human evolution retained certain traits. An interesting theory for that is that gays and lesbians can cooperate with their siblings to raise nieces and nephews, instead of competing with them for resources.
And individually would benefit from positive relationships with those nieces and nephews (financially, physically when old, socially), without having to directly bear the costs of having those kids either.
