Hindu Media Wiki

As per our scriptures any child born from a married woman will be considered as  legitimate irrespective of husband of the woman being the biological father of the child or not.

Every offspring of an unmarried woman will be treated as illegitimate.

Based on these rules Karan was illegitimate as Kunti was not married at the time Karan was born. For that reason Kunti abandoned him.

All the five Pandavs were considered legitimate as both Kunti & Madri were married at the time of their birth even Pandu was not the biological father of any of the five Pandavs. 

Even though Ved Vyas, biological father of both Pandu and Daratrastra was not Bharatwansi or Kaurav, but both were treated as legitimate heir of throne and bloodline as they were born from the widows (Ambika & Ambalika) of Vichitravirya .