A cool bit of Fridge Brilliance (See TVtropes.org for what this means) when thinking about Shaun's motivations:
If you read Shaun's terminal in his room as well as the projects he's banned research into in Advanced Sciences, you find out he's abandoned all research into cybernetics and that the people of the Institute are envious of Kellog's immortality but Shaun has nothing but disgust for it. He even talks about how "merging machines and humans" is something which disrupts the purity of humankind.
If you think about Shaun's loathing for cybernetics coming from Kellog then it makes sense. He knows Kellog is a monster and stole him from his parents, so he's naturally going to be bigoted against cybernetics and cyborgs. It also explains why he doesn't allow research into creating more people like Kellog despite the fact it could naturally extend the lifespan of Institute scientists (including himself).
It also explains why he considers the Synths to be machines rather than his children. As cyborgs, they're creatures like Kellog rather than himself. He was selected because of the "purity of his genome" and he must think Synths sully that sort of thing. He's able to overlook that with Synth Shaun because he's trying to recapture the life Kellog took away from him.
It does explain why Synths have almost no rights, though, even though its members seem willing to discuss the issue philosophically.