It's not about inequality but survival of a tribe. All things being equal, how could you not consider reproductive fitness? Yes, these were individuals, but they were also part of a starship crew on a lifelong mission. What kind of medical officer does not consider the long term impact on the ship and crew?
Ok, I know, they only have an hour to tell a story....
That argument would work under some conditions but not these. In this case:
A) The slowest they expected to get home was 70 years, which is not enough generations for long term generational attrition to come into play.
B) Exponential population increase would strain the carrying capacity of the ship, so maintaining equilibrium is more important than supercharging reproduction potential.
C) The death rate among crewmembers is a more imminent concern than future reproduction rates. Skillsets that keep crew alive is more important than future hypothetical reproduction odds.
D) It's the 24th century, if ship population really became a problem a generation or two down the road they have all kinds of artificial means to produce offspring.