Gut reaction #1: I don't know anything about John Robertson.
Gut reaction #2: John Robertson is probably not a libertarian.
Gut reaction #3: Diane Paul is definitely not a libertarian.
I plan to obtain a copy of the complete Paul paper, investigate John Robertson, then report my findings on this blog. I predict my 3 gut reactions are accurate. Also, for the record, preconception sex selection does not violate the plumbline libertarian premises (self-ownership, original appropriation, and non-aggression), therefore libertarian bioethics permits PSS; however libertarian bioethics does not mandate PSS, so opponents of PSS should not fear libertarian bioethics.