The Gurus did that out of reverence for Guru Nanak Dev Ji, the exact same reason Guru Gobind Singh Ji went to visit Ram Rai even though he was excommunicated. Even during the time of Guru Ram Das Ji he still believed it was impossible for people to find god in a household life. Sri Chand wasn't a child, Guru Nanak even made it known how Bhai Lehna was picked and why. Gurbani Still calls them evil hearted, the verses were uttered by Bhai Satta and Balwand, during the time of Guru Arjun Dev Ji. So there goes your stupid theory.
Having a physical Lineage doesn't suddenly change the deeds of anyone, even the Guru's own offspring were treated the same as his Sikhs as all Sikhs were his kids.
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