Jackie Robinson was instantly among the best players in the NL on his arrival in the majors. He was selected by the Dodgers from a short-list of “can’t miss” players.
Michael Sam is a prospective mid-round draft pick (#4 according to reports). Fourth-round Picks are expected to make their squads, but may not contribute right away.
Comparing sexual orientation to race is problematic. But to compare a star player with a middling prospect is also stretching things a bit.
There is, nor was there, ever a "can't miss" player. Actually, Robinson was a relatively low-profile player in the Negro Leagues and had only been playing professionally a relatively short time (no more than a year), when the Dodgers decided to sign him as the first black player in modern "organized baseball." They made him prove himself by playing at the highest minor league level with their Montreal farm team in the International League for one year, which he did, exceeding expectations. His selection as the first black player over fellow Negro League players was not based on pure baseball skills, but on his education, experience, and personality as well.