The head coach has to be impartial and dedicated to simply making the team be the best it can be in order to accomplish the ultimate goal of WINNING. How can she do that if she has a romantic relationship with one of her players? The other players would always be questioning her motives with respect to that player. This is not a little league or even high school endeavor where the coach's son (or daughter) gets all the playing time.
“The other players would always be questioning her motives with respect to that player.”
Exactly. You can’t have the head coaching boinking one of the players. It destroys morale.